Poultry Breeding and Production manual by Dr.Shamoil Tariq

UVAS Poultry Breeding
and Production
Department of Poultry Production
UNIVERSITY OF VETERINARY AND ANIMAL
SCIENCES, LAHORE-PAKISTAN

i
Poultry Breeding and Production
Table of Contents
Origin and Evolution of Domestic Chicken .............................................................................1
Heredity and Heritability..........................................................................................................4
Gene function and genetic code ..............................................................................................15
Quantitative Traits and Its Inheritance .................................................................................22
Inheritance of Qualitative Characters....................................................................................28
Additive Gene Action..............................................................................................................40
Breeding Systems ....................................................................................................................43
Selection and Selection Pressure.............................................................................................49
Reproductive problems as influenced by selection for higher body weight..........................59
Skeletal problems as influenced by selection for higher body weight ...................................64
Genetic resistance to diseases in Poultry ................................................................................67

1
Origin and Evolution of Domestic Chicken
By Dr. Jibran Hussain and Sohail Ahmad
The ancestor of the domestic chicken, red jungle fowl (Gallus gallus), can still be found today in regions
of South East Asia, including India, Burma, Malaysia, Thailand, and Cambodia. The first of these birds is
believed to have been domesticated over 8000 years ago. These jungle fowl are much smaller than most of
today’s domestic varieties, and are a tropical species adapted to live in much warmer conditions than most
current poultry. They are typically found in areas with thick vegetation befitting of their name.
The jungle fowl were initially domesticated for either religious or entertainment purposes, and are still used
in sacrificial religious ceremonies or for cockfighting. The Romans were the first society to develop and
use the chicken as an agricultural animal. Romans even developed specialized breeds including laying hens
that could lay an egg a day; a rate similar to today’s commercial lines. The Romans expanded from just
creating breeds of chicken to creating an entire poultry industry which utilized concepts such as force
feeding, hybrid vigor, and caponizing. However, with the fall of the Roman Empire, the industry collapsed
as well and chickens became little more than farmyard scavengers. Those original breeds created by the
Romans were lost and the practice of keeping poultry for agricultural purposes fell out of favor and did not
resume until the 19th
century.
In the 19th
century poultry breeding came back into prominence in the European region. As early as the
year 1810, six breeds of poultry are known to have existed in England (the Game, the White or English, the
Black or Poland, the Darkling, the Large or Strakeberg and the Malay). Over a fifty-year period in England,
formal poultry shows were developed and many new breeds were created leading to the formation of
numerous Breed Societies. The modern breeds we have today are mainly derived from two types of birds:
the Asiatic and the Mediterranean. Other breeds were developed by crossing breeds, and the geographic
origin of a breed is often indicated in their name (i.e., Rhode Island Red).
During the last two centuries more than 300 breeds of chickens have been developed; however, very few
of these breeds have survived commercialization and many breeds have been lost forever. The last 40 years
have seen the rise of the commercial hybrid rather than new breeds of chickens. These hybrids are of two

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main kinds; meat-type and egg-type. Both types of hybrids have been selected to maximize the amount of
production and minimize the amount of food needed for that production. Today less than two dozen private
companies worldwide create hybrids based on specific production characteristics (body weight, growth
rate, livability, pullet quality, age at sexual maturity, egg weight, egg production, eggshell quality, interior
egg quality, and ability to convert feed to eggs or meat).
Egg-laying types can be divided into light and medium hybrids. The light hybrids are derived from the
White Leghorn. These birds lay white eggs and the mature female weighs around 3.3 lbs. The medium
hybrids are derived from the Rhode Island Red and lay brown eggs with a mature female weighing around
4.4 lbs. Both types of hybrids have been highly selected to produce more and larger eggs while decreasing
their body size and feed intake. These two types of hybrids not only differ in their size, feather and egg
color but also their temperament. White hybrids are often better suited to be raised in cages, while brown
hybrids do well in cage-free systems.
The meat hybrids have been developed from breeds such as the Cornish and White Plymouth Rock. These
hybrids have been selected for growth rate, meat yield, and proportion of white meat as well as high feed
conversion. Today the majority of the meat lines are derived from the Cornish on the male side. The Cornish
gives the modern broiler chicken its broad breast, short legs, and plump carcass. The majority of meat lines
are also white feathered. White feathers are preferable as they are easier to pick than dark feathers, and do
not leave dark spots on the carcass. To ensure that offspring are white feathered, male lines are bred to be
dominant for white plumage. Even when these males are crossed with a coloured-feather female, the
majority of their offspring will have white, or nearly white, feathers. Another trait that has been selected in
the modern meat bird is yellow or white skin. This trait has been selected for solely because of consumer
preference.
Concepts and Methods of Poultry Genetic Selection
Single Line: Breeder uses a closed flock, continuously selecting the better birds each generation and
breeding from them.

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Hybrid Vigor: Outbreeding which results in increased quality of the hybrid offspring.
Male Line and Female Line: Crossing of a Male line only line with a Female online line. The opposite sex
of each line is destroyed at day 1 of age.
Strain Cross: Crossing of two or more different strains rather than selecting for traits within one strain.
Only a few desired traits are selected within each strain rather than all the desired traits in one strain.
Two-line Cross: Allows for offspring to have positive traits from both lines, however, at a lower rate than
the original lines.
Three-line Cross: First a two-line cross is done then the offspring of that cross are crossed with a third line
to obtain more positive traits.
Four-line Cross: Two separate two line crosses are completed than the offspring of these two crosses are
bred to obtain offspring with traits from all four lines.
Strains used for crossing must nick: This means that the two lines that are being crossed must complement
each other.
Inbred Crosses: Crossing of related individuals to improve uniformity. This often reduces performance.
Performance can be restored by crossing lines.
Sex-linked Meat Lines: Certain feather colours and speed of feather growth can be linked to the sex of the
bird. The trait will only appear in one of the sexes, making separation of chicks at hatch easier.

4
Heredity and Heritability
By Summera Mussarat
A heritable trait is most simply an offspring's trait that resembles the parents' corresponding trait.
Inheritance or heredity was a focus of systematic research before its inclusion as a key concept within
evolutionary theory. An influential 18th
and early 19th
century theory of heredity was pre-formation. This
view took several forms, each maintaining that organisms were passed on from one generation to the next,
miniature and yet fully formed, and development was simply the growth of the miniature organism.
Subsequent accounts of heredity included the theory that organisms inherited traits that their parents had
developed through response to various environmental pressures. This view was widely held during the
19th
century and usually attributed to Lamarck. A different concept of heredity was crucial to Darwin's view
that evolutionary change results from natural selection acting upon inherited traits under variation.
Weismann's experimental refutation of the inheritance of acquired traits paved the way for the combination
of Darwin and Mendel's views of the nature of heredity. The systematic study of heredity in the 20th
century
focused on the gene as the unit of heredity.
If the variation is owing entirely to environment and not at all to heredity, then the expression of
the character in the parents and in the offspring will show no correlation (heritability = zero). On the other
hand, if the environment is unimportant and the character is uncomplicated by dominance, then the means
of this character in the progenies will be the same as the means of the parents; with differences in the
expression in females and in males taken into account, the heritability will equal unity. In reality, most
heritabilities are found to lie between zero and one. Similarly, Twin studies as pointed out by darwins cousin
francis galton, monozygotic (MZ) twins are genetically identical, while dizygotic (DZ) twins share half of
their genes, same as any other siblings. However, MZ and DZ twins are often raised in very similar
environments, especially if of the same sex. Thus twins are naturally occurring experiments in the relative
effects of genetics and environment. –a further problem: DZ twins of the same sex often look quite different
they may not really get the same degree of similar treatment as MZ twins. The relevant statistic:
concordance. –if twins both have the disease, they are concordant. –if one has it and the other doesn’t, they

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are discordant. Traits with a large genetic component will show a higher concordance rate for MZ twins
than for DZ twins. For 100% genetic trait, should be 1.0 in MZ twins and 0.5 for DZ of same sex.
Concordance ratio: MZ / DZ. The higher it is, the more genetic a trait is.
Mendel’s Law
Mendel discovered that, when he crossed purebred white flower and purple flower pea plants (the parental
or p generation), the result was not a blend. Rather than being a mix of the two, the offspring (known as the
f1 generation) was purple-flowered. When Mendel self-fertilized the f1 generation pea plants, he obtained a
purple flower to white flower ratio in the f2 generation of 3 to 1. The results of this cross are tabulated in
the punnett square to the right.
He then conceived the idea of heredity units, which he called "factors". Mendel found that there are
alternative forms of factors—now called genes—that account for variations in inherited characteristics. For
example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white.
The alternatives “forms” are now called alleles. For each biological trait, an organism inherits two alleles,
one from each parent. These alleles may be the same or different. An organism that has two identical alleles
for a gene is said to be homozygous for that gene (and is called a homozygote). An organism that has two
different alleles for a gene is said to be heterozygous for that gene (and is called a heterozygote).
Mendel also hypothesized that allele pairs separate randomly, or segregate, from each other during the
production of gametes: egg and sperm. Because allele pairs separate during gamete production, a sperm or
egg carries only one allele for each inherited trait. When sperm and egg unite at fertilization, each
contributes its allele, restoring the paired condition in the offspring. This is called the law of segregation.
Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during
gamete formation. This is known as the law of independent assortment.
The genotype of an individual is made up of the many alleles it possesses. An individual's physical
appearance, or phenotype, is determined by its alleles as well as by its environment. The presence of an

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allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of
an inherited pair differ (the heterozygous condition), then one determines the organism’s appearance and is
called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called
the recessive allele. Thus, in the example above dominant purple flower allele will hide the phenotypic
effects of the recessive white flower allele. This is known as the law of dominance but it is not a transmission
law, dominance has to do with the expression of the genotype and not its transmission. The upper case
letters are used to represent dominant alleles whereas the lowercase letters are used to represent recessive
alleles.
In the pea plant example above, the capital "p" represents the dominant allele for purple flowers and
lowercase "p" represents the recessive allele for white flowers. Both parental plants were true-breeding, and
one parental variety had two alleles for purple flowers (PP) while the other had two alleles for white flowers
(pp). As a result of fertilization, the f1hybrids each inherited one allele for purple flowers and one for white.
All the f1 hybrids (Pp) had purple flowers, because the dominant P allele has its full effect in the
heterozygote, while the recessive p allele has no effect on flower color. For the f2 plants, the ratio of plants
with purple flowers to those with white flowers (3:1) is called the phenotypic ratio.
Mendel's laws of inheritance
Law Definition
Law of segregation
During gamete formation, the alleles for each gene segregate from
each other so that each gamete carries only one allele for each gene.
Law of independent assortment
Genes for different traits can segregate independently during the
formation of gametes.
Law of dominance
Some alleles are dominant while others are recessive; an organism
with at least one dominant allele will display the effect of the
dominant allele.

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Law of segregation (the "first law")
The law of segregation states that every individual contains a pair of alleles for each particular trait which
segregate or separate during cell division (assuming diploidy) for any particular trait and that each parent
passes a randomly selected copy (allele) to its offspring. The offspring then receives its own pair of alleles
of the gene for that trait by inheriting sets of homologous chromosomes from the parent organisms.
Interactions between alleles at a single locus are termed dominance and these influence how the offspring
expresses that trait (e.g. the color and height of a plant, or the color of an animal's fur). Book definition: the
law of segregation states that the two alleles for a heritable character segregate (separate from each other)
during gamete formation and end up in different gametes.
More precisely, the law states that when any individual produces gametes, the copies of a gene separate so
that each gamete receives only one copy (allele). A gamete will receive one allele or the other. The direct
proof of this was later found following the observation of meiosis by two independent scientists, the German
botanist Oscar Hertwig in 1876, and the Belgian zoologist Edouard van Beneden in 1883. Paternal and
maternal chromosomes get separated in meiosis and the alleles with the traits of a character are segregated
into two different gametes. Each parent contributes a single gamete, and thus a single, randomly successful
allele copies to their offspring and fertilization.
Law of independent assortment (the "second law")
In the inheritance of more than one pair of traits in a cross simultaneously, the factor responsible for each
pair of traits are distributed to the gametes.
The law of independent assortment, also known as "inheritance law", states that separate genes for separate
traits are passed independently of one another from parents to offspring. That is, the biological selection of
a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the
selection of the gene for any other trait. More precisely, the law states that alleles of different genes assort
independently of one another during gamete formation. While Mendel’s experiments with mixing one trait
always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with mixing

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two traits (dihybrid cross) showed 9:3:3:1 ratio. But the 9:3:3:1 table shows that each of the two genes is
independently inherited with a 3:1 phenotypic ratio. Mendel concluded that different traits are inherited
independently of each other, so that there is no relation, for example, between a cat's color and tail length.
This is actually only true for genes that are not linked to each other.
Independent assortment occurs in eukaryotic organisms during meiotic metaphase i, and produces a gamete
with a mixture of the organism's chromosomes. The physical basis of the independent assortment of
chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with
respect to the other bivalent chromosomes. Along with crossing over, independent assortment increases
genetic diversity by producing novel genetic combinations; of the 46 chromosomes in a
normal diploid human cell, half are maternally derived (from the mother's egg) and half are paternally
derived (from the father's sperm). This occurs as sexual reproduction involves the fusion of
two haploid gametes (the egg and sperm) to produce a new organism having the full complement of
chromosomes. During gametogenesis—the production of new gametes by an adult—the normal
complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can
join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as
those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy.
In independent assortment, the chromosomes that result are randomly sorted from all possible combinations
of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined
"set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can
end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of
gametes formed from maternal and paternal chromosomes will occur with equal frequency. For human
gametes, with 23 pairs of chromosomes, the number of possibilities is 223
or 8,388,608 possible
combinations.[4]
the gametes will normally end up with 23 chromosomes, but the origin of any particular
one will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic
variability of progeny.

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Law of dominance (the "third law")
Mendel's law of dominance states that recessive alleles will always be masked by dominant alleles.
Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the
dominant phenotype, while still having a heterozygous genotype. Law of dominance can be explained easily
with the help of a mono hybrid cross experiment: - in a cross between two organisms pure for any pair (or
pairs) of contrasting traits (characters), the character that appears in the f1 generation is called "dominant"
and the one which is suppressed (not expressed) is called "recessive." each character is controlled by a pair
of dissimilar factors. Only one of the character’s expresses. The one which expresses in the f1 generation
is called dominant. It is important to note however, that the law of dominance is significant and true but is
not universally applicable.
According to the latest revisions, only two of these rules are considered to be laws. The third one is
considered as a basic principle but not a genetic law of Mendel.
Mendelian trait
A Mendelian trait is one that is controlled by a single locus in an inheritance pattern. In such cases, a
mutation in a single gene can cause a disease that is inherited according to Mendel’s laws. Examples
include sickle-cell anemia, tay-sachs disease, cystic fibrosis and xeroderma pigmentosa. A disease
controlled by a single gene contrasts with a multi-factorial disease, like arthritis, which is affected by several
loci (and the environment) as well as those diseases inherited in a non-mendelian fashion.
Non-Mendelian inheritance
Mendel explained inheritance in terms of discrete factors—genes—that are passed along from generation
to generation according to the rules of probability. Mendel's laws are valid for all sexually reproducing
organisms, including garden peas and human beings. However, Mendel’s laws stop short of explaining
some patterns of genetic inheritance. For most sexually reproducing organisms, cases where Mendel’s

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laws can strictly account for the patterns of inheritance are relatively rare. Often, the inheritance patterns
are more complex.
What are half sibs and full sibs?
The individuals having one common parent are half sibs whereas the individuals having both parents
common are full sibs.
Variance
Variance is a measure of variation among any data set. Those differences are due to two factors:
1. Genetics (heredity)
2. Environment
All things except heredity come under the environment. Environmental proportion of variance or
differences is not transmitted to the next generation. The genetic makeup is transmitted to the next
generation.
To study the transmission of a character or trait to the next generation, concept of heritability was
introduced.
Heritability in broad sense:
It is the proportion of phenotypic variance to the total variance.
H2
=
𝜎2H
𝜎2P
Difference in genetic variance is mainly due to different gene actions:
1. Additive gene action
2. Dominance
3. Epistasis

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Each gene has its own contribution to the genotype and cumulative effect of all genes constitute a phenotype
e.g. skin colour.
Differences in phenotypes due to differences in genotypes are known as additive genetic variation.
Quantitative traits are mostly under the influence of additive genetic variance.
Heritability in narrow sense:
Proportion of phenotypic variance due to additive genetic effects of genes is called heritability in narrow
sense. (h2
) is symbol for heritability in narrow sense.
h2
=
𝜎2A
𝜎2P
If character is mostly depending on environment, then its heritability is low. If fertility is low, then mostly
it is environmental problem. Growth characters are mostly under the influence of additive gene action.
Growth characters can be improved through selection. If the heritability of any character is higher then we
will make progress through selection.
Selection
“It is the differential reproductive rate”. Or
“Preferences of certain individuals over the others to produce next generation are called selection”.
Correlation
It is a term which describes how the two variables tend to move together. It is usually denoted by r.
correlation may be positive or negative. It ranges between -1 to +1 through 0.
When two variables move in the same direction the correlation is said to be positive. Sometimes variables
move in opposite direction then the relationship between such variables is termed as negative correlation.
In correlation we do not consider independence or dependence of any variable. Correlation has not any
unit.

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Regression
It is a term which describes a unit change in one variable as a result of unit change in the other
variable. Unit use for regression is “b”. In regression one variable will be dependent and other
will be independent. e.g. relationship between cause and effect.
Estimation of h2
half sibs and full sibs’ correlation:
 Relationship between full sibs will be 50%
 Relationship between half sibs will be 25%
 Relationship between identical twins will be 100%
We can divide variance into different components through ANOVA:
SOV d f S.Sq M.Sq Ex M.Sq
Between sires (n-1)
3 - 1 = 2
B 𝐵
2
= D 𝜎2
w + K 𝜎2
s
1. Within sires
2. Error
3. Residual
Total df – Between df
(N-1)-(n-1)
14 – 2 = 12
A-B = C 𝐶
12
= E 𝜎2
w
Total 15 – 1 = 14 A
SOV: Source of variance
d f : Degree of freedom
S Sq: Sum of squares
M Sq: Mean squares
Ex M Sq: Expected mean squares
𝜎2
w: Within variance

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𝜎2
s: Within sires
K: number of offspring per cock or sire or average number of progeny per sire
n: number of sires
Correction Factor (C.F)
C.F =
(Σ𝑋𝑖)2
𝑁
Total Sum of Squares (T.S.S) = ΣXi2
– C.F = A
Between Sum of Squares (B.S.S) =
Σ𝑆𝑖2
𝐾
− 𝐶. 𝐹 = B
Residual mean square represents within variance 𝜎2
w.
Between mean square represents this whole term 𝜎2
w + K 𝜎2
s.
Intra sire correlation = t =
𝜎2S
𝜎2w+𝜎2S
Heritability (h2
) = 2 (t) for full sibs
h2
= 4 (t) for half sibs
h2
= 1 (t) for identical twins
If data are on full sibs, then multiply with 2.
If data are on half sibs, then multiply with 4.
If data are on identical twins, then multiply with 1.
If the heritability of any trait is high, it means that phenotype for that trait is under the influence of
additive gene action.
Selection on the basis of individuals will be more effective if the heritability is high.

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So we can conclude that:
 Knowledge of heritability is needed to formulate effective breeding plan for improvement in next
generation.
 Knowledge of heritability determines that whether the trait under consideration is controlled by
additive gene action or non-additive gene action.
 Heritability is also useful for the decision that how much emphasis ne given for selection or other
manage mental practices.
Table 1. Heritability estimates of different traits in poultry
Layer Broiler
Trait Heritability % Trait Heritability %
Egg shape
60 6 week body weight 45
Egg weight 55 Total feed consumption 70
Adult body weight 55 6 week feed conversion 35
Shell texture 25 Breast fleshing 10
Albumen quality 25 Fat deposition 50
Body depth 25 Dressing % 45
Age at sexual maturity 25
Keel length 20
Overall egg production 15
Blood spot 15
Adult livability 10
Hatch of fertile 10
Chick livability 5
Fertility 5

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Gene function and genetic code
By Muhammad Usman
Genetics
Genetics is the study of genes, heredity, and genetic variation in living organisms. It is generally considered
a field of biology, but it intersects frequently with many of the life sciences and is strongly linked with the
study of information systems.
The father of genetics is Gregor Mendel, Mendel studied 'trait inheritance', and patterns in the way traits
were handed down from parents to offspring. Trait inheritance and molecular inheritance mechanisms of
genes are still a primary principle of genetics in the 21st century, but modern genetics has expanded beyond
inheritance to studying the function and behaviour of genes.
Genome
The total DNA in an organism is the genome of that individual. Genome is present in the form of
chromosomes, DNA and genes. For example, in human being, there are 46 chromosomes in 23 pairs having
total genetic information.
DNA and Nucleotides
Deoxyribonucleic acid (DNA) is the molecular basis of genes. DNA molecules is a long helical structure.
Each strand is called polymer because it is composed of many repeated units called nucleotides. A
nucleotide is composed of nitrogenous base (purine or pyrimidine) linked to a sugar which are linked with
phosphoric acid molecule.
DNA is composed of a chain of nucleotides, i.e., adenine (A), cytosine (C), guanine (G), and thymine (T).
Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence
along the DNA chain. DNA normally exists as a double-stranded molecule, coiled into the shape of a double
helix. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A
pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all
necessary information.

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RNA
Ribonucleic acid (RNA) is a polymeric molecule implicated in various biological roles
in coding, decoding, regulation, and expression of genes. It differs with DNA by containing the sugar ribose
instead of deoxyribose found in DNA. It’s a single stranded structure. It contains uracil instead of thymine,
thus adenine always joins with uracil.
Cellular organisms use messenger RNA (mRNA) to convey genetic information (using the letters G, U, A,
and C to denote the nitrogenous bases guanine, uracil, adenine, and cytosine) that directs synthesis of
specific proteins. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as
a complementary language.
Transcription
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA
(mRNA) by the enzyme RNA polymerase. During transcription, a DNA sequence is read by an RNA
polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript.
Translation
In translation, messenger RNA (mRNA)—produced by transcription from DNA—is decoded by
a ribosome to produce a specific amino acid chain, or polypeptide. The polypeptide later folds into
an active protein and performs its functions in the cell.
Genes
Smallest biological unit of inheritance. A portion (or sequence) of DNA that codes for a known cellular
function or process (e.g. the function "make melanin molecules"). A single 'gene' is most similar to a single
'word' in the English language. The nucleotides (molecules) that make up genes can be seen as 'letters' in
the English language.
Genetic Codes
Genetic code is the set of rule by which information encoded in genetic material in translated into protein
by living cells. This information is in the form of triplet codon. It is first transcribed into RNA than into

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protein. Every triplet codon is specific for specific amino acid. There are 64 possible combinations of
nucleotides i.e., 43
= 64. They are all written in 5’ to 3’ direction.

18
61 codes for 20 amino acids. Three (UAA, UGA, UAG) are stop codons that terminate translation. One
start codon (AUG) codon for methionine. Protein synthesis starts with Methionine in eukaryotes and
formylmethionine in prokaryotes. More than one codon can code for each amino acid. Each amino acid can
be coded by more than one codon.
There are 6 main features of genetic codes
 Triplet codon
 Specificity
 Degeneracy
 Universality
 Non-overlapping
 Punctuated
1. Triplet Codon
Genetic codes are always in the form of triplet codon, made up of three nucleotide base pairs.
2. Specific
Each nucleotide base pair in the genetic code specified for specific amino acid. For example, UUU specified
for Phenylalanine.

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3. Degenerate
A given amino acid may be coded for more than one codon. For example, GUU, GUC, GUA, and GUG all
are responsible for the production of Valine. Similarly, Lysine is produced by AAA and AAG. This property
of genetic codes allows for possible mutations to be less damaging.

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4. Universality
The genetic codes are universal in all organisms except mitochondria and protozoan nuclear DNA. For
example, AGA is specific for the production of Arginine in all organisms in which genetic codes has been
studied.

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5. Non-overlapping
The genetic codes are non-overlapping. No base of a given triplet contributes to the part of code of adjacent
triplet.
6. Punctuated
The codon is punctuated in the form of start and stop codon. Excluding start and stop codon, the actual code
determining the sequence of amino acid is unpunctuated.

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Quantitative Traits and Its Inheritance
By Ali Salman Ajmal
Quantitative Traits
Characters such as egg production, egg weight, body weight etc. can be measured on a definite scale and
hence are called as quantitative traits.
Quantitative Traits Vs Qualitative Traits
Nicking
Since the quantitative traits are controlled by polygenes, it is possible to develop lines which are
homozygous dominant for several but different factors. When these lines re crossed, the offspring often
perform better than either the parents. This property of the parents to combine and produce offspring
superior to themselves is called nicking; e.g., an efficient breeder can develop suitable lines which can nick
and produce commercial chicks of high productivity.
Threshold characters
Certain polygenic traits do not show continuous variability phenotypically. A classical example for this
being hatchability of eggs- until the total genetic, environmental and interaction influences, if any, which
represents the phenotypic potential, is enough for the chick to hatch, the hatching doesn’t occur. That means
QUANTITATIVE TRAITS QUALITATIVE TRAITS
Traits are measured, rather than counted.
Traits are more often qualitative, at best they can
be counted.
Traits show a continuous variation. Traits are all-or-none type, no variability.
Traits are controlled by polygenes, each having a
minor effect, minor genes.
Traits are more often controlled by one or few
genes having a major effect, major genes.
Expression of the trait is often influenced by the
environment.
Expression of the trait is independent of the
environment.
Phenotypic classes do not reflect the genotypic
classes.
Phenotypic classes, on most occasions are good
indicators of the genotypes.

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there is a threshold value below which the chick fails to hatch and above the value, it hatches. Genotypically,
there may be a distribution of threshold values of each of the embryos in the population but, phenotypically,
the expression is binomial. In other words, the cause is a continuous variable whereas the effect is discrete.
Such polygenic traits referred to as “threshold or all-or-none traits”. Phenotypic potential of such traits can
not be measured in an individual, but if coded suitably, it can be estimated from a group of individuals by
arithmetic means of the phenotypes.
Component Characters
Many quantitative and threshold traits are mathematically and/or biologically determined by two or more
contributing traits. For example – egg production is largely determined by clutch size, pause duration, age
at sexual maturity rate of lay. Similarly body weight of broilers is determined by FCR, rate of gain, body
conformation and others. Each of these contributing traits is a polygenic trait and therefore study of these
contributing traits help in understanding the traits to the component trait under consideration.
Idealized Population
Most often, animal breeder is primarily interested in improving the productive efficiency of domestic
animals and hence is dealing with quantitative traits. Therefore it is required that a population in which no
forces are operating to alter its genetic structure (referred to as idealized population) is studied so that the
actual effects of manipulations by the animal breeder can be quantitatively predicted.
Gene and genotypic frequencies of quantitative traits
Considering ‘n’ number of alleles at a locus, number of possible genotypes is n (n+1)/2 and proportion of
individuals belonging to each of the genotypes is referred to as ‘’genotypic frequency’’ whereas proportion
of each allele out of all the genes at that locus gives the ‘’gene frequency’’.
Assuming a locus A with alleles ‘A’ and ‘a’ with complete dominance, whose frequencies are p an q,
respectively, in an idealized large population, genotypic frequencies are:
(Pa + qa)×2 = p×2AA +2pq Aa + q×2 aa

24
And the phenotypic are Dominant allele, A = p×2 + 2pq
And recessive allele, a =q×2
In case of quantitative traits, the alleles will have equal effects and assuming p=q= 1/2 and that gene action
is additive with the addition of ‘A’ gene to the phenotype causing one unit increase in phenotype whereas
that of ‘a’ gene causing no change in the phenotypic value, the phenotypic values will be : AA=2, Aa=1 ,
and aa=0, whose frequencies in an idealized population will be p×2= ¼, 2pq=1/2 and q×2 =1/4,
respectively.
Under realistic situations, most of the quantitative traits are controlled by more than two pairs of alleles but
exact number cannot be known. Neither equal allelic frequency at each locus need be always true nor their
effects equal and additive.
Sex-linked genes
In case of sex-linked genes, equilibrium is not achieved in one random mating but at each generation,
difference in the gene frequency between the sexes is halved. In other words, if an allele is more frequent
in female at the beginning, in the subsequent generation, it will be more abundant in males.
Phenotype of an individual
The phenotype of individual (P) is determined at the moment of conception when its genotype (G) at all
loci affecting the trait is fixed by union of gametes produced by its parent at random. The influence of G
on P depends on effects of and interaction among the alleles each locus concerning the trait. The P is also
influenced by the environmental factors (E) ever since the conception itself till the time the trait is actually
measured. The interaction between G and E (GE) also influences the P of the individual.
Therefore P= G+E+GE

25
Breeding Value (BV)
BV of an individual is the value of the individual judged by the mean value of its progeny. An individual
contributes 50% of genes to its progenies produced over several mating and if each gene has an average
affect of ‘a’, the total effect contributed is ∑a. However as only half the genes of its progeny is contributed
by the individual, the BV= 2∑ a. and hence BV is the property of both gene frequency as well as the
population. In other words BV is a transmitting ability of an individual and it is twice the phenotypic value
of individual inheriting one allele from a parent of that genotype, the other coming at a random from
population.
Heritability (h2
)
Heritability is the ratio of additive genetic variances, V (A), to V (P). It indicates the proportion of V (P)
attributable to V (A) or is the proportion of differences among individuals expected, on average of the
population, to be transmitted to their progeny.
Characters of h2
1. Since it is a ratio of variances, is always =0. Further as the animal breeder is not interested
in traits whose V (A) =0, heritability is always greater than 0.
2. Numerator is a part of the denominator in the formula for calculating it, therefore h2
= 1.
3. It is not a biological constant. The numerator V (A) is influenced by number of loci, type
of gene action, average affect of gene and gene frequency. The denominator is determined
by the magnitude of variances due to dominance, epistasis and environment.
4. h2
for a given trait is not constant between populations because of differences in the
variance components.
5. Within a population, h2
can change over a period of time due to changes in any or many of
the components of both the numerator and the denominator.

26
Estimated h2
Values (%)
Repeatability
Certain traits are expressed more than once in a lifetime, for example egg weight. These traits are repeatable
traits and this is called as repeatability defined as a proportion of V (P) for a trait, attributable to permanent
differences among individuals. Traits which are repeated may be influenced by the same genes each time
they are expressed and so-called permanent environmental influence.
Repeatability is not a constant value, it varies with the trait, for the same trait in different populations and
the same population at different times. It is used in estimating lifetime productivity of individuals, and as
upper limit of h2
because it is very easy to compute unlike heritability.
Characters h2
Layer
Fertility, Chick livability 5
Adult livability, Hatchability (FES) 10
Egg production, blood spots, Broodiness 15
Keel length 20
Age at sexual maturity, body depth, shell texture, albumen quality 25
Adult body weight, egg weight 55
Egg shape 60
Broiler
Breast fleshing 10
7-week feed conversion 35
7-week broiler weight, dressing percentage 45
Fat deposition 50
Total feed consumption 70

27
Correlations
In practical situations, many quantitative traits are associated with each other. For example, body weight
with egg weight, age at sexual maturity with egg production.
Types of correlation
1. Phenotypic Correlation
This measures linear association between two traits. It predicts deviation from population mean in one
trait of an individual as a function of its deviation from the population mean of the other.
2. Genetic correlation
It measures the extent to which the same genes are grossly linked genes, cause simultaneous variation
in two different traits or the extent to which individuals genetically above average in one trait are
genetically above, equal or below average for a second trait.
3. Environment correlation
If the same environmental effect causes simultaneous variation in both traits, a correlation results which
is referred to as environmental correlation. For example, increased light during growing period causes
early sexual maturity, increased egg production, and reduced egg weight.

28
Inheritance of Qualitative Characters
By Sohail Ahmad
Traits that cannot be assigned a straight forward number or value like height or number of fingers are called
as qualitative traits e.g., eye colour, hair colour, etc. The phenotype can be observed, which means that it
can be classified, counted or weighed. For instance, a hen’s phenotype for laying ability can be seen by the
number of eggs it produces. This is also the trait, as it is expressed ‘in the phenotype’. The list of phenotypes
which can be recorded is infinite.
Three different types of traits can be observed in the phenotype:
1. Qualitative
2. Quantitative
3. Threshold traits
A qualitative trait is expressed qualitatively, which means that the phenotype falls into different categories.
These categories do not necessarily have a certain order. The pattern of inheritance for a qualitative trait is
typically monogenetic, which means that the trait is only influenced by a single gene. Inherited diseases
caused by single mutations are good examples of qualitative traits. Another is blood type. The environment
has very little influence on the phenotype of these traits.
Some qualitative traits like plumage, skin and eye colour of domestic fowl have an aesthetic appeal. They
are also of economic importance to the commercial poultry industry.
Threshold Traits
A threshold trait is a trait, which is inherited quantitatively, but is expressed qualitatively. Normally a
lot of genes form the basis of a threshold trait, which is why it should be treated as a quantitative trait. A
common characteristic of threshold traits and Mendelian traits is that they occur family wise. When dealing
with diseases with low population frequency it is not possible to differentiate between a Mendelian inherited

29
disease and a threshold disease. In both cases the frequency of the disease will be much higher in close
relatives of an affected animal, than the frequency in the population.
The Integumental and Ocular Pigments
Pigments of chicken include two important types:
1- Melanins
2- Carotenoids
Melanins:
1- They are responsible for feather coloration and the dark pigments of skin and connective tissue.
2- Most wide spread class of natural pigments in living organisms.
3- On the basis of difference in colour, chemical composition, solubility properties and associated
pigment granule structure Melanins are divided into two types;
a- Eumaelanin, is the pigment of black or blue feathers, the eyes, skin and connective tissues.
b- Pheomelanin, is to be deposited only in red-brown, salmon and buff coloured feathers.
Carotenoids:
1- Carotenoids, xanthophyll, give the yellow coloration to the skin and egg yolk.
2- The main carotenoid of chicken is the yellow fat-soluble pigment called Xanthophyll.
3- The yellow colour of the egg yolk, body fat, skin, shank and beak is due to xanthophyll.
4- Males and non-laying females store xanthophyll in body fat, skin and blood, but laying female utilize
all of their ingested xanthophyll in the production of egg yolk.
5- The relationship between egg laying, and the presence of xanthophyll in the skin and its derivatives,
has been used as an indicator of the egg production status of hen.
The two types can also interact with each other and other cell types, to produce a variety of shank and eye
colours, as well as structural sheen in the plumage.

30
Vascular Contributions
The heme pigment of blood also contributes to coloration of the eye and the skin and its derivatives.
Genetics of Plumage Colour
Although both melanins and carotenoid pigments contribute to the feather colour of certain avian species,
it is the melanin that determines the plumage colour and pattern of the domestic fowl.
The ultimate presence and distribution of the melanins is complicated by difference in feather form and
structure associated with age and sex as well as structural variations between and within traits and individual
feathering.
Genetics of Pigmentation in Skin, Eyes and Other Non-Feather Tissues
1- Pigmentation of non-feather tissues involves the carotenoids, as well as the melanins.
2- The combination of the two lead to varying phenotypes, most obvious in yellow, white, green and
blue shank colours.
3- The deposition of both types of pigments in non-feather tissue is also influenced by both specific
individual genes and unidentified polygenic modifiers.
Autosomal white skin
1- The major determiners of xanthophyll deposition in the skin are the autosomal white (W+) and
yellow (w) allele.
2- White skin is considered to be wild type, since W+ is present in the red jungle fowl.
3- The white skin genes prevent the transfer of xanthophyll into the skin, beak and shanks, but the iris
of the eye, yolk and blood serum contain normal amounts.
4- The white and yellow skin phenotypes cannot be separated accurately until the chicks are 10-12
weeks of age.

31
Sex-linked white skin
A sex-linked recessive mutation y also eliminates xanthophyllic pigment from the shank and the skin.
Yellow head
An autosomal recessive mutation (g) associated with enhanced yellow coloration of the vascularized facial,
comb and wattle tissues. It is apparent at few weeks of age but is more striking prior to the onset of sexual
maturity.
Ear lobe colour
Most breeds have red ear lobe, although breeds of the Mediterranean Class, including Leghorn, Minorca
and Spanish have enamel-white ear lobes. The nature of white pigment is neither carotenoid nor melanin,
but a compound made up of purine bases.
The presence of the white purine pigment appears to be inherited as a polygenic trait.
Polygenic Trait
A trait that is controlled by a group of non-allelic genes. Polygenic traits are controlled by two or more than
two genes (usually by many different genes) at different loci on different chromosomes. These genes are
described as polygenes. Examples of poultry polygenic inheritance are height, skin colour and weight.
Polygenes allow a wide range of physical traits. For instance, height is regulated by several genes so that
there will be a wide range of heights in a population.
Expression and Interaction of Genes
If each gene were expressing itself in a separate test tube, it would be reasonable to expect all genes to be
functionally independent but these genes are not in separate test tubes; they are located on the chromosomes
present in the same nuclei of the same cell. So it can be expected that an allele of a gene pair may have

32
influence on the expression of the second (non-allelic) gene and sometime it may alter the expression of
the one or more non-allelic genes. Such influences are called as interaction of genes.
A classic example of gene interaction based on the results of crosses between different breeds of poultry
has been reported in early part of twentieth century by William Bateson and his associate R.C. Punnett
(after whom the Punett Square was named). They were working on to test and verify the findings of
Mendel’s work after its discovery in 1900. They had chicken and garden pea plants for the experimentation.
Wyandotte Breed has a comb known as the “rose comb”, Brahmas and some other varieties “pea” comb,
Leg horns have single combs; all of these are breed true. But cross between rose combed and single combed
showed that rose was dominant to single and there was a segregation into three fourth (3/4) rose and one
fourth (1/4) single in F2.
Parents ♂ Rose x ♀ Single
RR rr
F1 Rr All Rose (Hybrid)
F2 Rose ¾ : Single ¼
In crosses between Pea combed and single combed birds, Pea was dominant over single and a 3:1 ratio
appeared in the F2. An interesting result was however, obtained when Rose combed birds were crossed with
Pea-combed birds. For the F1 birds a new comb called walnut comb, which has been a characteristic of
another breed known as Malay breed of fowls having no relationship with the birds, or breed from which
now walnut combed bird have been produced.

33
Parents ♂ Rose ♀ Pea
RRpp x rrPP
Gametes (Rp) (rP)
F1 RrPp Walnut
Inter-se mating ♂ RrPp x ♀ Rr Pp
Gametes (RP), (rP), (Rp) (rp)
F2
RP Rp RP rp
RP RRPP
Walnut
RRPp
Walnut
RrPP
Walnut
RrPp
Walnut
Rp RRPp
Walnut
RRpp
Rose
RrPp
Walnut
Rrpp
Rose
rP RrPP
Walnut
RrPp
Walnut
rrPP
Pea
rrPp
Pea
rp RrPp
Walnut
Rrpp
Rose
rrPp
Pea
rrpp
Single
Walnut 9; Rose 3; Pea 3; Single 1
When walnut combed birds (F1) were bred together, in F2 generation there appeared walnut, rose and pea
combed fowls but single combed ones as well. The ratio was 9:3:3:1.
This ratio is expected in F2 from a cross of parents differing in two genes. The doubly dominant class in
F2 was walnut, whereas the numbers of singles obtained indicated that this type contained both the recessive

34
genes involved. The crossing of these single combed fowls producing singles supported it. The walnut
comb depends on the presence of two dominant genes R and P. One of these genes alone (R) produces the
rose comb; the other alone (P) produces the Pea comb. The combination of the recessive alleles of these
genes produces the single type of comb.
The mode of inheritance of the genes for rose and pea does not differ at all from usual Mendelian
inheritance. The differences that distinguish this and similar cases from ordinary dihybrid inheritance are
that (1) The F1 resembles neither parent and (2) apparently novel characters appear in F2. One of these new
characters (walnut) evidently results from an interaction between two different dominant genes and the
other (single comb) results from the interaction of their two recessive alleles. The peculiarities are due not
to a new method of inheritance but simply to the circumstances that both genes involved happen to express
themselves in the same part of the organism, for example in this case, the comb.
Genes “R” and “P” were non allelic, but each was dominant to its allele (i.e. ‘R’ over ‘r’ and ‘P’ over ‘p’).
When ‘R’ and ‘P’ were together, as in F1 (RrPp), the two different products interacted to produce Walnut
comb. The two non-allelic genes ‘R’ and ‘P’ acted independently in different ways, similar to the ways in
which codominant alleles act.
Epistasis
One important type of functional interaction between different genes occurs when an allele or genotype at
one locus “masks” or inhibits the expression of a non-allele or genotype at a distinct locus; such an
interaction is called epistasis.
1. The ability of one gene to mask the phenotype derived from a second gene (non-allelic).
2. Genetic effects due to interactions among two or more pairs of non-allelic genes.
3. Epistasis may also be defined as the interaction of two or more pairs of genes that are not alleles. These
may be interactions between genes on the same chromosome or on different chromosomes.

35
The genes those suppress the expression of non-allelic genes are called Epistatic genes whereas the genes
whose action is suppressed are called Hypostatic genes.
Epistasis should not be confused with dominance. Epistasis is the interaction between different genes
(non-allelic) whereas dominance is the interaction between different alleles of the same gene. The
modifications of Mendelian ratio are mostly due to epistatic effects of genes.
Modified Segregation Ratio 9: 7
In sweet pea plants there are two true breeding varieties having white flowers. When these varieties were
crossed the results were as under:
Parents White × White
CCpp ccPP
Gametes (Cp) (cP)
F1 CcPp Purple
Inter-se mating CcPp × CcPp
Gametes (CP), (Cp), (cP), (cp)
CP Cp cP cp
CP CCPP
Purple
CCPp
Purple
CcPP
Purple
CcPp
Purple
Cp CCPp
Purple
CCpp
White
CcPp
Purple
Ccpp
White
cP CcPP
Purple
CcPp
Purple
.ccPP
White
.ccPp
White
cp CcPp
Purple
Ccpp
White
.ccPp
White
.ccpp
White
The phenotypic ratio was purple 9: white 7

36
Each parent in this cross was from a different true breeding varieties of sweet pea having white flowers.
The colourlessness in flowers is due to the presence of different recessive genes and the presence of both
dominant genes was necessary for the coloration. Any one pair in recessive homozygous condition is
responsible for discoloration. It is an example of double recessive epistasis
Modified Segregation Ratio 9: 3: 4
Three coat colours are present in mice (1) Grey often called Agouti (2) Albino and (3) Black. Albino
always breed true and this condition has been found to behave as a simple recessive to any colour. Black is
recessive to agouti and breeds true. When black mice are crossed with ordinary albinos, the progeny are
all agouti like the wild type. When these agoutis are inbred their progeny consist on an average 916 agouti,
316 black and 416 as albinos.
These results are explained on the assumption that the parents differ in (1) a gene, C, necessary for
the development of any colour, which the black mice contain but which is lacking in the albinos; and in (2)
a gene for the agouti pattern, A, which results in a blending of the black hairs with yellow. Since the black
mice cannot contain this gene, A, it must have come from the albino parent, where, in the absence of the
ability to develop any colour at all, it could have no visible expression. The recombination of these two
genes, one for colour and the other for the agouti pattern, reconstitutes the genotype of the wild type i.e.,
agouti.
Parents Black × Albino
CCaa ccAA
Gametes (Ca) (cA)
F1 CcAa Agouti
Inter-se mating CcAa × CcAa
Gametes (CA), (Ca), (cA), (ca)

37
F2
CA Ca cA ca
CA CCAA
Agouti
CCAa
Agouti
CcAA
Agouti
CcAa
Agouti
Ca CCAa
Agouti
Ccaa
Black
CcAa
Agouti
Ccaa
black
cA CcAA
Agouti
CcAa
Agouti
ccAA
albino
ccAa
albino
ca CcAa
Agouti
Ccaa
Black
ccAa
albino
ccaa
albino
Phenotypic ratio recessive epistasis
Agouti Black Albino
9 : 3 : 4

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Modified Segregation Ratio 12: 3: 1
In summer squash fruit here are two true breeding varieties regarding coat colours of fruit i.e. White and
Green. White colour is dominant over green. When White coloured plants were crossed with green coloured
plants, all the progeny were having White coloured fruit. But on inter-se mating for F2 generation, there
appeared three phenotypes with the ratio, White 12: Yellow 3: and Green, 1.
Parents White × Green
YYGG × yygg
Gametes (YG) (yg)
F1 YyGg White
Inter-se mating YyGg × YyGg
Gametes (YG), (Yg), (yG), (yg)
YG Yg yG yg
YG YYGG
White
YYGg
White
YyGG
White
YyGg
White
Yg YYGg
White
Yygg
White
YyGg
White
Yygg
White
yG YyGG
White
YyGg
White
yyGG
Yellow
yyGg
Yellow
yg YyGg
White
Yygg
White
yyGg
Yellow
yygg
Green

39
The phenotypic ratio was white12: yellow 3: green 1. Each parent in this cross was from a different true
breeding varieties of squash fruit having white or green fruit. The colourlessness in coat colour is due to
the presence of one dominant gene i.e., ‘Y’ and the other dominant gene ‘G’ only expresses its effect by
producing yellow colour of fruit in the absence of the first dominant gene i.e., when that is in homozygous
recessive condition (yy). When both the gene pairs are in homozygous recessive condition, colour of the
fruit is green. It is an example of dominant epistasis
13: 3 dominant recessive; 15: 1 double dominant; 6: 3: 3: 4 double recessive
Pleiotropy
In general, one gene affects a single character, but many cases are known, where a gene influences more
than one character (trait). Such genes are known as pleiotropic genes and the condition is termed as
pleiotropy. In pea plants, the same gene that affects flower colour, also influences the colour of seed coat
and colour of leaf axial. In cotton, the gene that affects the fiber character also affects plant height, boll size
and fertility. In human beings, a recessive gene in homozygous condition produces sickle cell hemoglobin
resulting in Sickle cell anemia condition. This gene in recessive form also produces other effects like mental
retardation, poor physical development, paralysis, heart attack, rheumatism, kidney damage and failure.
Lethal Genes
A gene that causes death of an organism or a cell possessing it in proper arrangement for expression is
known as a lethal gene. Lethal genes may be dominant and exert their effect in heterozygotes. Such genes
are comparatively rare and difficult to study since they are rapidly eliminated from a population unless their
effects occur late in life after affected individuals have produced offspring. Most lethal genes are recessive
and exert their effects only when homozygous.
A lethal gene may have its effect any time from the formation of the gamete un-till birth or shortly
afterwards. In other words, death of the organism may occur at any stage of development immediately
following fertilization, during embryonic differentiation, at parturition, or post-natal.

40
Sub Lethal or Semi-Lethal
The genes, which cause death of the young after birth or sometime later in life, are called sub-lethal or
semi-lethal genes.
Detrimental Genes
Some other genes do not cause death, but definitely reduce viability or vigor. They may cause some
anatomical changes. These are referred to as non-lethal or detrimental genes. They are also known as
deleterious genes. They have undesirable effects on the individual’s viability or use fullness either in
homozygous or heterozygous state.
Non-Additive Gene Action
Non additive gene action refers to the non-linear expression of genes. This means that the addition of a
gene to the genotype does not add an equal amount to the phenotype. For example, where dominance is
complete, the heterozygote, Aa is not midway between AA and aa, but it is approximately the same as AA.
This would be a non-linear phenotypic expression. If genotype Aa were midway between AA and aa, this
would be linear for this one pair of genes. Several different kinds of non-additive gene action are known,
e.g. dominance and recessiveness, incomplete dominance dihybrid inheritance and Epistasis etc.
Additive Gene Action
It is another kind of gene expression. The non-allelic genes affecting the same trait in such a way that each
adds to the effect of others in the phenotype. Such genes are called additive genes and the phenomenon as
additive gene action.
In this type of inheritance, there is no sharp distinction between genotypes, but many gradations between
the two extremes. Skin colour inheritance in human being can be a good example of additive gene action.

41
P1 Black skin white skin
AABB × aabb
F1 AaBb Medium color
Inter se mating
F2 1 AABB Black
2 AABb Dark
1 AAbb Medium
2 AaBB Dark
4 AaBb Medium
2 Aabb Light
1 aaBB Medium
2 aaBb Light
1 aabb White
Five different phenotypes are observed with a continuous gradation between white and black and no sharp
distinction between only two classes. The proportion of offspring of different skin colours in the F2
generation would be:
1 black
4 dark
6 medium
4 light
1 white

42
Genes A and B are called contributing genes, because they make a contribution to the darkening of the skin.
Genes a and b are called neutral genes because they contribute nothing or at least very little, to skin colour.
An important aspect of additive gene action is that none of the genes is dominant or recessive. Each
contributing gene adds something that makes the skin darker in colour, and the effects of each gene
accumulate. Hence, the term additive gene action.
Additive gene action is thought to affect most traits in farm animals that are of economic importance.
Growth rate, milk production, conformation, carcass quality, as well as other traits are affected by this type
of inheritance. Because many different pairs of genes may affect such traits, such traits are polygenic.

43
Breeding Systems
By Shamoil Tariq and Sohail Ahmad
Breeding is an application of genetic principles to improving hereditary for economically important traits
in domestic animals. Animal breeding is the genetic principle for the improvement of farm animals. The
aim of the breeders is to evolve outstanding and improved types of animals which can render better services
to man. Selection and system of breeding constitute the only tolls available to the breeder for improvement
of animals, since new genes cannot be created, though they can recombine into more desirable groupings.
System of breeding have been broadly divided as under:
 Inbreeding: Breeding of the related animals
 Outbreeding: Breeding of the unrelated animals
Inbreeding
This means the mating of related individuals. Each animal has two parent, four grandparents, eighth great
grandparents biologically and so on, having 1,024 ancestors in the 10th
generation and 10,48,576 ancestors
in the 20th
generation. Relationship, therefore, becomes a vague term and must be specified. So in order to
be more specific we should say that inbreeding involves the mating of related individuals within 4-6
generations. It has also been defined as the mating of the more closely related individuals than the average
of the population.
Inbreeding can again be divided into following groups:
Inbreeding
Close breeding
Sie to daughters
Son to Dam
Full brother and sister
Line breeding
Half-brother and sister
or mating of animals
mroe distantly related,
e.g., cousing mating

44
Breeding Systems
Close breeding
This means the mating of full sister to full brother or sire to his daughter or dam to her son. These types of
mating should be used only when both parents are outstanding individuals, and then only at increased risk
of bringing undesirable recessive genes into homozygous form in the progeny.
Advantages of Close breeding
 Undesirable recessive genes may be discovered and eliminated by further testing in this
line.
 The progeny is more uniform than outbred progeny.
Disadvantages
 The undesirable characteristics are intensified in the progeny if unfavourable gene
segregation occurs.
 It has been observed that the progeny becomes more susceptible to diseases.
 Breeding problems and reproductive failure usually increase.
 It is difficult to find out the stage of breeding at which it should be discontinued in order
to avoid that bad effects of the system.
Line breeding
This means that mating of animals of wider degrees of relationship than those selected for close breeding.
It promotes uniformity in the character. Homozygosity is not reached so quickly as in close breeding.
Neither desirable nor harmful characters are developed so quickly. It is a slowed method for the fixation of
hereditary outstanding bull or cow and the progeny is mentioned as being line bred to certain ancestors.
Advantages of Line breeding
 Increased uniformity
 The dangers involved in close breeding can be reduced

45
Breeding Systems
Disadvantages of Line breeding
 The chief danger in line breeding is that the breeder will selected the animal for pedigree
giving no consideration to real individual merit. This may in some cases result in a few
generations which receive no benefits from selection.
Out-breeding
Out-breeding is the breeding of unrelated animals and this involves the following types of breeding:
1. Out-crossing
2. Cross breeding
a. Criss-crossing
b. Triple crossing
c. Back crossing
3. Species hybridization
4. Grading-up
Out-crossing
It consists in the practice of mating of unrelated pure breed animals within the same breed. The animals
mated have no common ancestors on either side of their pedigree up to 4-6 generations and the offspring
of such a mating is known as the out-cross.
Advantages
 This method is highly effective for characters that are largely under the control of genes
with additive effect, e.g., milk production, growth rate in beef cattle, etc.
 It is an effective system for genetic improvement if carefully combined with selection.
 It is the best method for most herds

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Breeding Systems
Cross breeding
It is the mating of animals of different breeds. It is generally used where the crossed progeny is directly
marketed and are not needed for breeding and further multiplications. It has become quite common in pigs
and in the production of hybrid chickens. With beef cattle also it is practised to a certain extent. Cross
breeding for milk production has been tried with varying degrees of success. It is generally used for the
production of new breeds.
Methods of cross breeding
Criss-crossing. When the two breeds are crossed alternatively, the method is known as criss-crossing. This
method is proposed for utilising heterosis in both dams and progeny. Breed A females are crossed with
breed B sires. The cross-breed females are mated back to sires of breed A and so on. In this system the
cross breeds soon come to have about 2/3 of their inheritance from the breed of their immediate sire with
1/3 from the breed being used.
A A 50
B B 50 A 75
A B 25 A 37.5
B B 62.5 A 68.5
A B 31.5
Triple-crossing. In this system three breeds are crossed in a rotational manner. It is also known as rotational
crossing.
Three breeds are used in this system. The females of crosses are used on a sire of pure breeds in rotation.
The crossbreds will soon come to have 4/7 of inheritance of the breed of immediate sire, 2/7 form the breed
of maternal grand-size and 1/7 of the hereditary material of the other pure breed.

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Breeding Systems
A
A 50 A 25
B 50 B 25 A 63
B C C 50 B 12 A 31
A C 25 B 57 A 15
B C 12 B 28 A 58
C C 57 B 14 A 28
A C 28 B 58
B C 14
Back Crossing. Back crossing is mating of a crossbred animal back to one of the pure parent races which
were used to produce it. It is commonly used in genetic studies, but not widely used by breeders. When one
of the parents possesses all or most the received traits, the back cross permits a surer analysis of the genetic
situation than F3 does.
Heterosis or Hybrid Vigour
Heterosis or hybrid vigour is a phenomenon in which the cross of unrelated individual often result in
progeny with increased vigour much above their parents. The progeny may be from the crossing of strains,
breeds, varieties, or species. One of the explanation for this increased vigour is that genes favourable to
reproduction are usually dominant over their opposites. As a species or breed develops, it becomes
homozygous for some dominant genes, bur some undesirable genes are also present at high frequencies.
When on breed is crossed with the other, one parent supplies a favourable dominant gene to offset the
recessive one supplied by the other and vice versa.

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Breeding Systems
Grading up
Grading is the practice of breeding sires of a given breed to non-descript female and their offspring for
generation after generation.

49
Selection and Selection Pressure
By Urooj Khan
Selection is an important tool for changing gene frequencies to better fit individuals for a particular purpose.
The aim of selection is to make the animals and birds of the next generation as good as or better than those
of the present one. It is very generally believed that our present breeds of domestic animals have been
slowly evolved from the breeds that have gone before and ultimately from wild ancestors by a process of
selection.
Example
If we see that the birds in a breed of poultry vary in size if we weigh or measure them carefully it seems
evident that when we select the smallest pair of bird in our selected line will get smaller and smaller
regularly until finally we have reduced them to a bantam replica of the breed with which we started. The
only things that are passed from parent to offspring are material substances in the cells the genes which
certainly help to give the developing animal certain qualities but which are by no means to be looked upon
as the sole causes of the inherited characters.
Types
Selection is of two general kinds,
1. Natural (or that due to natural forces)
2. Artificial (or that due to the efforts of man)
Natural selection
In nature the main force responsible for selection is the survival of the fittest of a particular environment.

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Example
Some of the most interesting cases of natural selection are those involving man himself. All races of man
that now exist belong to the same species because they are inter fertile or have been in all instances where
mating have been made between them. All races of man now in existence had a common origin and at one
time probably all men had the same kind of skin pigmentation which kind we have no sure way of knowing.
As the number of generations of man increased, mutation occurred in the genes affecting pigmentation of
the skin causing genetics variations in this trait over a range from light to dark or black. Man began to
migrate in to the various parts of the world and lived under a wide variety of climatic conditions of
temperature and sunshine. In Africa it is supposed the dark skinned individuals survived in larger numbers
and reproduced their kind because they were better able to cope with environmental conditions in that
particular region than were individuals with a lighter skin.
Recently evidence has been obtained that there may be a differential selection for survival among humans
for the A, B and O blood groups. It has been found that members of blood group A have more gastric cancer
than other types and member of type O have more peptic ulcer. This would suggest that natural selection is
going on at the present time among these different blood groups and the frequency of the A or O genes
might be gradually decreasing unless, of course there are other factors that have opposite effects and have
brought the gene frequencies into equilibrium. Natural selection is a very complicated process and many
factors determine the proportion of individuals that will reproduce. Among these factors are differences in
mortality of the individuals in the population especially early in life differences in the duration of the period
of sexual activity the degree of sexual activity itself and differences in degree of fertility of individuals in
the population.
Two types of natural selection:
Sexual selection
Charles Darwin first suggested sexual selection. Animals compete with others of the same species for the
chance to mate.

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Diversifying selection
Animals are varying the other animals in a same species due to the action of different climatic condition.
In a hypothetical population grey and white rabbits are better able to blend with a rock environment than
white rabbits resulting in diversifying selection.
Artificial selection
Artificial selection is selection practiced by man. It may be defined as the efforts of man to increase the
frequency of desirable genes or combinations of genes, in his herd or flock by locating and saving for
breeding purposes those individuals with superior performance or which have the ability to produce
superior performing offspring when mated with individuals from other lines or breeds.
There are two approaches or types of artificial selection. First is the traditional "breeder’s approach" in
which the breeder or experimenter applies "a known amount of selection to a single phenotypic trait" by
examining the chosen trait and choosing to breed only those that exhibit higher or "extreme values" of that
trait. The second is called "controlled natural selection," which is essentially natural selection in a controlled
environment. In this, the breeder does not choose which individuals being tested "survive or reproduce," as
he or she could in the traditional approach.
Example
Artificial selection can produce dramatic changes in the field of evolution and agriculture. For example, a
racehorse having desirable traits is allowed to mate with a healthy female horse in order to ensure that the
offspring produced would have the desired excellent racing traits. Similarly, breeding fruits and vegetables
of good size and produce together are done with the hope of getting a similar produce. Superior quality of
wheat, corn, soybeans have been grown through careful breeding in agriculture. Similarly, hybrid varieties
of orchids and roses have been cultivated by the process of artificial selection. Thus it can result in an
increased yield of milk by cows and better quality meat of various poultry and cattle.

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Disadvantages of Artificial Selection
Artificial selection can create organisms that are dangerous to the population. For example, killer bees, also
known as Africanized honey bees, were bred to produce more honey, but have killed a couple of people
due to the unforeseen trait of increased aggressiveness being passed on. In some cases, like the banana,
artificial selection has led to sterility.
Artificial selection also removes variation in a population (Nature of Life).
 Can create new mutations.
 Can perpetuate harmful mutations.
 Very expensive.
 Can cause harm to the animals that are being bred such as a shorter life span.
Selection pressure
The extent to which organisms possessing a particular characteristic are either eliminated or favored by
environmental demands. It indicates the degree of intensity of natural selection. Selection pressure can be
regarded as a force that causes a particular organism to evolve in a certain direction. It is not a physical
force, but an interaction between natural variation in a species and factors in its environment that cause a
certain form to have an advantage over the others. This can be thought of as a “pressure” that pushes the
evolution of that organism toward a greater prevalence of this variation.
Methods of selection
Several methods may be used for determining which animal should be saved and which should be rejected
for breeding purposes.
 Tandem method
 Independent culling method

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 The selection index
 QTL
Tandem method
In this method selection is practiced for only one trait at a time until satisfactory improvement has been
made in this trait. The efficiency of this method depends a great deal upon the genetic association between
the traits selected for. If there is a desirable genetic association between the traits so that improvement in
one by selection results in improvement in the other trait not selected for the method could be quite efficient.
If there is little or no genetic association between the traits which mean that they are inherited more or less
independently, the efficiency would be less than if the traits were genetically associated in a desirable
manner. Since a very long period of time would be involved in the selection practiced the breeder might
change his goals too often or become discouraged and not practice selection that was intensive and
prolonged enough to improve any desirable trait effectively. A negative genetic association between two
traits in which selection for an increase in desirability in one trait results in a decrease in the desirability of
another, would actually nullify or neutralize the progress made in selection for any one trait. The efficiency
of such a method would be low.
Independent culling method
In the use of this method selection may be practiced for two or more traits at a time but for each trait a
minimum standard is set that an animal must meet in order for it to be saved for breeding purposes. The
failure to meet the minimum standard for any one trait causes that animal to be rejected for breeding
purposes.
Example
To show the disadvantage of this method let us use in swine where minimum standards are set so that any
pig saved for breeding must be from a litter of 8 weaned must weigh 180 pounds at 5 months of age and

54
must have no more than 1.3 inches of back fat at 200 pounds live weight. Let us assume that pig A was
from a litter of 9 pigs weaned weighed 185 pounds at 5 months and had 1.3 inches of back fat. For pig B
let us assume that it was form a litter of five weaned weighed 225 pounds at five months and had 0.95 inch
of back fat at 200 pounds. If the independent culling method of selection were used pig B would be rejected
because it was from a litter of only five pigs. However, it was much superior to pig A in its weight at five
months and in back fat thickness and much of this superiority could have been of a genetic nature. The
independent culling method of selection has been widely used in the past especially in the selection of cattle
for show purpose where each animal must meet a standard of excellence for type and conformation
regardless of its status for other economic traits. It is also used when a particular colour or colour pattern is
required. It does have an important advantage over the tandem method in that selection is practiced for
more than one trait at a time.
The selection index
This method involves the separate determination of the value for each of the traits selected for and the
addition of these values to give a total score for all of the traits. The animals with the highest total score are
then kept for breeding purposes. The influence of each trait on the final index is determined by how much
weight that trait is given in relation to the other traits. The amount of weight given to each trait depends
upon its relative economic value since all traits are not equally important in this respect and upon the
heritability of each trait and the genetic associations among the traits. The selection index is more efficient
than the independent culling method for it allows the individuals that are superior in some traits to be saved
for breeding purposes even though they may be slightly deficient in one or more of the other traits. If an
index is properly constructed taking all factors into consideration it is a more efficient method of selection
than either of the other two which have been discussed because it should result in more genetic improvement
for the time and effort expended in its use. Selection indexes seem to be gaining in popularity in livestock
breeding.

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QTL
A quantitative trait locus (QTL) is a section of DNA that correlates with variation in a phenotype (the
quantitative trait). The QTL typically is linked to, or contains, the genes that control that phenotype. QTLs
are mapped by identifying which molecular markers correlate with an observed trait. This is often an early
step in identifying and sequencing the actual genes that cause the trait variation. Quantitative
traits are phenotypes that vary in degree and can be attributed to polygenic effects, i.e., the product of two
or more genes, and their environment.
Example
Several genes factor into determining a person's natural skin colour, so modifying only one of those genes
can change skin colour slightly or in some cases, such as for SLC24A5, moderately. Many disorders
with genetic components are polygenic, including dislocation of the hip, heart defects, neural tube
defects, cancer, diabetes, epilepsy, hypertension, thyroid diseases and numerous others.
Typically, QTLs underlie continuous traits (those traits that vary continuously, e.g. height) as opposed to
discrete traits (traits that have two or several character values, e.g. red hair in humans, a recessive trait, or
smooth vs. wrinkled peas used by Mendel in his experiments).
Moreover, a single phenotypic trait is usually determined by many genes. Consequently, many QTLs are
associated with a single trait. A quantitative trait locus (QTL) is a region of DNA that is associated with a
particular phenotypic trait. These QTLs are often found on different chromosomes. Knowing the number
of QTLs that explains variation in the phenotypic trait tells us about the genetic architecture of a trait. It
may tell us that plant height is controlled by many genes of small effect, or by a few genes of large effect.
Factor affecting the gene frequency
The following are some factors responsible for such variations.
 Selection

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 Gene mutations
 Migration
 Genetic drift
Selection
This is a very important factor that may be responsible for changes in gene frequencies. With respect to
farm animals selection means that individuals possessing certain desirable traits are caused or allowed to
produce the next generation. Thus individuals of a certain genotype may be retained in larger numbers for
breeding purpose than others. This causes an increase in the frequency of some genes and a decrease in the
frequencies of others. To illustrate how selection may change gene frequencies assume that we have a herd
of 100 shorthorn cows consisting of 25 red 50 roan and 25 white. In this herd the frequency of the red and
white genes would be equal 0.50 each. Let us assume that for some reason we decide to cull and sell all of
the white individuals. Selling these would leave 25 red and 50 roan and would increase the frequency of
the red gene 0.667 and decrease the frequency of white to 0.333. By culling the white and roan individuals
in this herd the frequency of the white gene would be reduced to 0 and that of the red gene would be
increased to 1.00.
Gene mutations
A gene mutation is due to a change in the code sent by the DNA molecule by means of RNA to the
ribosomes in the cytoplasm where amino acids are assembled into proteins. Mutations occur and have
occurred in genes affecting any trait whether determined by a single gene or by polygenes. Mutated genes
may occur at a high frequency in one population and at a low frequency or even may be absent in another.
Example
Purebred Hereford cattle are red with a white face and this is known as the Hereford pattern. The white face
gene appears to be dominant in Herefords. Solid faced purebred Herefords are seldom seen but the white

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on the face does vary from a complete white to one possessing patches of red especially around the eyes.
Pure bred black angus on the other hand are black with a solid coloured face. Thus the frequency of the
white face gene is near zero in angus. Most but not all angus are black although red angus are sometimes
produce by black parents. The purebred black angus breed is a good example of how the frequency of two
different genes may vary within the same breed. One of the distinguishing characteristics of angus cattle is
that they are polled (hornless). Polledness in angus appears to be a dominant trait. And the gene for
polledness p appears to have a frequency of close to 1.0 in this breed. Purebred angus do sometimes have
scurs which are loose horny projections from the head but scurs appear to be due to a separate gene SC in
this breed.
Genes differ in their rate of mutation. Some mutate more often than other. For example, the mutation rate
seems relatively high for certain recessive traits in human such as thalassemia and sickle cell anemia in
which a large proportion of the individuals in a population may be homozygous recessive. Other traits such
as albinism and pseudo hypertrophic muscular dystrophy appear to have a much lower mutation rate ranging
between 25 and 60 per million per generation.
A point should be reached however before the original gene is eliminated where selection in one direction
to eliminate one gene will finally equal the mutation rate in the other so that the gene frequency in the
population would become stable that is in equilibrium. This point where the elimination of a gene becomes
equal to its replacement by mutation is called mutation equilibrium. Mutation equilibrium may be reached
theoretically for another reason. It is well known that mutation rates are reversible in that there is not only
a mutation from gene A to a but only from a back to A again.
Migration
This may be other factor responsible for changes in gene frequencies in a population. Suppose we have a
population of humans in which the frequency of the M blood type gene is 0.915. In another population
some miles away the frequency of this same gene is 0.215. If individuals from the first group migrated to

58
the second and they intermarried the frequency of the M gene in the children would average somewhere
between 0.215 and 0.915 depending upon how much intermarriage occur.
Genetic drift
This is another factor that may be responsible for changes in gene frequencies and may result in a type of
genetic variation with in a population that is not due to natural selection. It is a random statistical fluctuation
independent of nature selection that sometimes is also referred to as the sampling nature of inheritance.
Wright has called this mechanism effecting frequencies random genetic drift. Suppose that in a large
population of humans the frequency of M gene was 0.60 and that of the N gene were 0.40. Let us also
assume that a small group of people left the larger group and migrated to a mew country to settle in a remote
area where they were isolated from other humans. It is possible that frequency of the M gene in this small
group might be very low and that of the N gene very high or vice versa just because of chance due to a
small population coming from a larger one.
Genetic drift may also be important in the development of inbred lines of livestock where a parent-offspring
or a brother-sister mating system is followed for several generations. Population size is responsible for the
importance of genetic drift in changing the frequencies of genes in a population. In a small population a
gene might either disappear or become fixed in a few generations because the population is so small that
even a slight change in the number of people carrying a gene might cause a large change in the percentage
of the total population showing the gene or not showing it. In a large population a change in the frequency
of a gene would affect a smaller percentage of the population. In a small isolated population inbreeding
increases from necessity whereas in a larger one it is often decreased. In a small population the chance
segregation of genes in the gametes and recombination in the zygote may be the main cause of genetic drift.

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Reproductive problems as influenced by selection for higher body weight
By Rao Muhammad Kashif Yameen
Need of selection
Selection of heavy weight poultry breeders is necessary to gain the following parameters:
 Achieve health and high carcass value
 Feed efficiency
 Feed conversion ratio
 Birds come into production at the right time
 Birds produce the optimal number of eggs
 Birds produce fertile, high quality, hatch-able eggs
 Economics
Selection for meat
The main goal of selection for broiler production is to maximise the growth potential of the bird, resulting
in high body weight and high breast meat yield. However, both of these objectives are negatively correlated
with fertility. As a result, the fertility of broiler breeders has become a challenge. Problems arising from
parent breeding stock and broiler chicks can lead to regional loss in the market and take considerable time
and effort to rectify. It is for this reason that broiler breeding programmes should focus not only on growth
and performance, but also on the reproductive performance of the parents, such as egg number, fertility and
hatchability.
Selection of breeders for high and efficient egg output per hen per year while for poultry meat production,
it has become clear that meat output per breeder has become a more dominant indicator of success. It is
known that the fertilizing ability of roosters from commercial broiler breeders is declining continuously
with each generation. Fertility and hatchability are major parameters of reproductive performance which
are most sensitive to environmental and genetic influences of heavy weight poultry breeders. Fertility refers

60
to the percentage of incubated eggs that are fertile while hatchability is the percentage of fertile eggs that
hatch. Fertility and hatchability are major parameters of reproductive performance which are most sensitive
to environmental, genetic influences and fating of bird.
Heritability estimates for fertility and hatchability in chickens range from 0.06-0.13, this indicates that the
non-genetic factors have a higher influence on these traits. Fertility refers to the percentage of incubated
eggs that are fertile while hatchability is the percentage of fertile eggs that hatch. Fertility and hatchability
are interrelated heritable traits that vary among breed, variety and individuals in a breed or variety. A
number of other factors including egg age.
Genetic selection for growth parameters in meat type chickens gives rise to a parent stock (broiler breeders)
that tends to lack the ability to self-regulate feed intake, as such, their high body mass is associated with
excessive fat deposition, reproductive disorders, and high mortality rates.
Ad libitum feeding of broiler breeder hens had significantly lower egg production as well as higher body
weight, plasma triglycerides, and cholesterol concentrations. Broiler breeder hens became heavy after
overfeeding, and high glucose availability due to hyper-phagia could result in lipo-toxicity and ovarian
dysfunction. Lipo-toxicity is associated with excessive accumulation of triacylglycerol, and fatty acids in
non-adipose tissues as well as altered circulating and tissue lipid profiles.
Industry Problems
Broiler breeder industry facing a major problem has often been the dramatic decline in fertility in male
birds particularly gaining heavy weight more than recommended or when breeders becomes old. Decline
in semen quality, testosterone production, number of sperms, semen volume, poor physical condition and
leg problems have been cited with excessive weight breeders. Fertility very poor due to both sex’s males
and females however it is maximum associated with males. In industry force molting of hens is an
economical practice which enhances the productive and reproductive life span of the birds.

61
It is clearly concluded that heavy weight breeders both males and females have negative effects on egg
production, fertility and hatchability. Reproductive problems can be minimizing at farm level by adopting
the following points.
 Artificial insemination
 Restricted feeding
 Feed management after peak of lay
 Sexual synchronization
 Inadequate Males
 Forced mating
 Spiking
 Molting
 Rear cockerels and hens separately at first
 A large sample of males, weighed weekly is necessary for an accurate assessment of current body
weight.
Problems associate with hens
It is generally assumed that if the hen is capable of producing eggs and the sperm is viable, then fertilisation
will occur. Infertility may be due to an absence of semen in the oviduct, mostly as a result of problems with
mating frequency or mating success are also affected by higher weight breeders. Studies in females indicate
that selection for higher body weight results in greater proportions of mature ovulations, and that defective
egg syndrome will result in a lower fertility and hatchability. This is influenced by a variety of factors, such
as genetic selection and management of breeders.
The female contributes to fertility through mating receptivity and spermatozoa storage in special sperm host
glands in the oviduct. This was demonstrated by where broiler line genetic selection proceeded on nutrient-
dense broiler diets while typical lower protein and energy rearing diets were used for parent stock.

62
Evidently, inadequate CP (amino acid) nutrition prior to photo stimulation, higher body weight of female
leads to poor persistency of fertility. Reproductive processes in females are the result of controlled
interaction between the hypothalamus-pituitary and the ovary, and are influenced by environmental,
selection or nutritional effects and having negatively relationship with higher weight gain. With
recommended weight gain and feed restriction in broiler breeder females is the normal ovary weight, the
number of yellow follicles during lay, normal eggs and multiple ovulations, higher weight hens having
more mature ova then normal hens and also produce double yolk eggs. It is concluded that unrestricted
access to feed leads resulted in heavy weight and lay low egg production rate and fewer suitable eggs for
incubation, observed that disturbances in follicular growth, differentiation, and ovulations in could be
attributed to changes in the steroid-producing capacity and in the sensitivity of the follicles to locally
produced growth factors in interaction with each other and with gonadotrophins.
Heavy weight female breeders also change at the ovarian level because heavy weight breeders with ad
libitum feeding have many mature ova also changes in the concentrations of luteinizing hormone (LH) and
follicle-stimulating hormone (FSH) that is important factors explaining the alterations in follicular
development and ovulation between poultry breeders. Some problems of heavy weigh hens are following
in points
 Excessive follicular development that can lead to oviduct prolapse
 Increased number of double-yolk eggs
 Egg yolk peritonitis (presence of egg yolk in the abdominal cavity)
 Erratic oviposition (laying outside the normal laying time)
 Laying more than one egg per day (with poor quality shells)
 Overweight hens may have poor fertility due to sperm transport problems in the oviduct
 They also may become too large to mate unsuccessfully

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Excess body weight and natural mating
In several commercial flocks experiencing depressed fertility a significant negative relationship was
reported between male body weight and both mating activity, fertility and hatchability. In broiler breeder
flocks, fertility may be negatively affected by males fed ad libitum because they have a reduced success
rate in natural mating. Full-size males can become obese and as a result suffer from foot and skeletal
problems making males unwilling or unable to mate with hens. Study reported a sharp decline in fertility
after 54 week of age and gain high body weight. The males were unable to mate due to physical
abnormalities such as excessively high body weight and leg problems. Previous study found male body
weight to have a greater effect on mating ability and therefore fertility than the occurrence of leg lesions.
In broiler breeder males, feed restriction reduced the amount of weight the legs had to support, resulting in
effective mating and greater fertility.
Male weight Management
It has long been clear that feed restriction to control body weight is both obligatory and beneficial in broiler
breeders to gain recommended fertility and hatchability rates, however, excessive feed restriction of males
during part or all of the growing period has been associated with decreased early fertility. The major impetus
for sex-separate feeding during the breeding period was the observation that poor fertility was associated
with overweight males and separate feeding was believed to be necessary to control male body weight.
With the advent of high nutrient density diets and more recently restricted feeding, a breeder flock can be
maintained economically for a longer period of time and at higher fertility and natural mating by male
breeders, during the grower period of broiler breeder flocks, restriction of feed in order to gain optimal
body weight is a common industry practice. Body weight control benefits the reproductive performance of
both female and male broiler breeders. The flock benefits from increased egg production, increased fertility,
hatchability, egg quality, reduced double yolk or malformed eggs, and reduced mortality due to body weight
restrictions. Broiler breeder male’s semen quality and quantity is related to close monitoring of male body
weight throughout the flock. Male body weight influenced the percentage of males producing semen.

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Skeletal problems as influenced by selection for higher body weight
By Jamil Kamran
Race for high growth heritability between commercial broiler breeding companies resulted in intense rise
in growth potential over the last 40 years. Heritability for growth capacity in modern broiler and meat type
turkey has been improve from its origin. As growth rates have escalated, skeletal problems revealing
themselves primarily as leg disorders have become increasingly prevailing in broilers and, to a lesser extent,
in turkeys. Leg disorders are to be found worldwide in modern broilers production. Studies in Denmark and
Sweden have observed high levels of leg deformities in broiler chicken Flocks. Different studies point out
that that one of the major causes of leg health problems in broiler chickens is high body weights due to fast
growth rate which may be genetically determined. There is a probability to separate between major skeletal
genes that express their effect distinctly, modifier genes that may or may not modify the effect of a major
gene, and the combinations of several to many genes interacting with the environment that are sometimes
termed the genetic background genes. A study showed a list and description of many of the genes
influencing development of the skeleton. Mutants of major skeletal genes are often harmful to the individual
and disappear from the population if their effects are dominant, but recessive mutant genes may remain. A
particular situation exists for the creeper (Cp) gene. Birds heterozygous for that gene have a shortening of
the long bones resulting from delayed cartilage replacement, whereas homozygotes for Cp die as embryos.
Some breeders of fancy strains like the creeper hens and therefore the gene remains in a few breeds Another
mutant with a major effect is the sex-linked dwarf gene, used by a number of the worldwide broiler breeding
companies. A number of genes governing the secreting of enzymes, cytokines and hormones may be
classified as modifying genes in relation to skeletal development and some of these may have very localized
activity. Mutants of these genes may cause a change in intensity of secretion, time of secretion, tissue
specificity, etc. Such mutants may have a positive, negative or neutral effect on the trait at a given time
compared with the wild type gene. The genetic correlations between body weight and incidences of crooked
toes, bow-leggedness and valgus deformity were estimated to be +0.22, +0.26 and +0.10, respectively.
After pooling all three skeletal disorders into one trait, the genetic correlation between disorders and body

65
weight remained strongly positive (+0.25). In contrast other study used a multiple logistic model, found
minimal genetic correlation between body weight at 3 or 6 weeks of age and either Varus or Valgus
angulation. There are some contradictory results relating TD and body weight. Studies on line differences
in growth capacity and TD have been performed on ducks with a clear indication of higher TD incidence
in the faster-growing lines. A similar relationship has been seen in male turkeys, though not in females.
“Twisted leg” (valgus-varus or angular bone deformity) is a common cause of leg problems. The growth
deformity is characterized by bowed or knock-kneed legs. It is estimated that 0.5-2% of broiler chickens in
normal flocks have bone deformities, with occasional cases where 5-25% of male broiler chickens are
affected in problem flocks. The deformity may occur in five-day-old chicks, but is more often seen when
the birds are 1-4 weeks old or older. Turkeys may also suffer from angular deformities, and some may have
problems standing. In typical, high stocking density environments, fallen birds may be trampled and
bruised, and some may seek refuge from being stepped upon by hiding under the feeders. As body weight
accumulates, deformities can lead to a slipped tendon or spontaneous fracture. Whether moderate or severe,
affected birds have difficulty walking and may hobble, preferring to sit. More severely affected birds are
unable to rise and may walk or creep on their hocks. These crippled birds may not be able to access feed
and water.
Tibial dyschondroplasia (TD), an abnormal mass of cartilage at the growth plate of a bone, usually the tibia,
is another common cause of leg problems. The end of the tibia may become enlarged, deformed, and
weakened, and the bone may bend backward as it grows. This can cause necrosis, spontaneous fracture,
severe lameness and in some cases, the complete inability to stand. Like birds suffering from severe twisted
leg deformities, crippled and unable to rise, some birds with severe TD can move only by creeping on their
hocks and may be unable to reach food and water.
Sources differ in describing the prevalence of TD. In 1995, researchers estimated that, in extreme cases, the
incidence of TD could reach 30-40%, especially for flocks of male birds. Aviagen, a leading breeding

66
company, has worked to reduce the incidence of TD, and its own figures show a reduction from
approximately 8% in 1999 to a projected level of less than 2% by 2005.However, studies published in 2001
and 2003 report elevated cases in common commercial strain broiler chickens, with a mean prevalence of
approximately 57% (animal welfare). While TD may be relatively common in broiler chickens and turkeys
raised for meat, it is rare or absent in other types of birds. Rupture of the gastrocnemius tendon that runs
along the back of the leg is a common problem in heavy broiler chickens. It is caused by excessive weight
on weak tendons, a consequence of rapid growth. If one leg is affected, the added stress may cause rupture
of the tendon in the other leg. Discoloration may be seen on the back of the legs due to hemorrhage. At six
weeks, broiler chickens have such difficulty supporting their abnormally heavy bodies that they spend 86%
of their time lying down. Combined with poor litter condition, the immobility of the birds leads to increased
incidence of painful contact dermatitis-breast blisters, hock burns, and foot pad lesions.
Findings suggest that the more integrated the description of leg health is, the more unfavourable is the
genetic correlation with growth. Selection of birds for good leg health from existing genotypes, and possible
trials of new bird genotypes by broiler breeding companies, could result in reduced levels of leg health
problems in the medium to long term. Genotype is known to be a factor in determining leg weakness and
for tibial dyschondroplasia. Genotype also affects walking ability.

67
Genetic resistance to diseases in poultry
By Ali Husnain
Genetic Resistance
Due to modern systems of management, usually with high poultry densities, diseases are able to
readily spread. As poultry is getting proliferated pathogens are also altering themselves in a different
ways. Any kind of genetic changes in the body of a host or a pathogen either actively or passively
responsible for the less or no chances of occurrence of disease is called as genetic resistance and it is
caused by non-hereditary risk factors. The combination of new methodology that enables the efficient
production of genetically-modified (GM) animals with exciting new tools to alter gene activity makes
the applications of transgenic animals for the benefit of animal (and human health) increasingly
likely.
Poultry
It refers to birds that people keep for their use and generally includes the chicken, turkey, duck, goose,
quail, pheasant, pigeon, guinea fowl, pea fowl, ostrich, emu and rhea.
Disease
It is a condition in which an individual shows an anatomical, chemical or physiological deviation from the
normal.
Disease is a problem!
Infectious disease adversely affects livestock production and animal welfare, and often has major impacts
upon human health and public perception of livestock production. For example, the costs of existing
endemic diseases are estimated to be 17% of the turnover of livestock industries in the developed world
and 35% to 50% in the developing world. Furthermore, epidemics, particularly in developed countries,
incur further major costs and have profound impacts upon the rural economy and on public confidence in
livestock production. In the United Kingdom (UK), direct evidence of this assertion can be seen in the cases
of bovine spongiform encephalopathy and the recent foot and mouth disease virus (FMDV) outbreak.
Disease affects the sustainability and competitiveness of a community, and animal disease also has many

Genetic resistance to diseases in poultry 68
effects on human health, for example: food safety is compromised when food is contaminated with
infectious pathogens and with chemicals used to control and treat infections – antibiotic resistance may
arise from the excessive and inappropriate use of antibiotics – zoonotic infections are often harboured by
domestic livestock (as is seen in the global effect of influenza).
Disease arises as a consequence of infection in the host animal by an infectious agent. Infectious agents
include entities such as prions for transmissible spongiform encephalopathies; microparasitic pathogens
such as viruses, bacteria or protozoa; or macroparasites such as helminths or nematodes. To control disease
effectively requires an understanding of the host-pathogen interaction for that particular disease.
Current major advances in gene transfer technologies, in parallel with the significant new genetic information
provided by genomic technologies, have made it possible to investigate the host-pathogen interaction in more
detail than ever before. Major advances in our understanding of disease are likely to be achieved within the
next few years. These technologies will lead to new opportunities for diagnosis, intervention and the selective
breeding of animals for resistance. The combination of advanced gene transfer technologies and traditional
disease control measures, should allow for more effective and sustainable disease control.
Importance of Poultry Industry
The poultry is one of the important components of the farmer's economy. It provides additional income and
job opportunities to a large number of rural populations in the shortest possible time. Poultry farming has
assumed much importance due to the growing demand of poultry products especially in urban areas because
of their high food value. It also involves small capital investment and provides useful employment to a large
number of people.
Types of Genetic Resistance
There are two types of genetic resistance as follows:
Vertical or Specific Resistance
Specific resistance of a host to the particular race of a pathogen is known as vertical resistance. This type
of resistance is governed by one or few genes.

69
Genetic Resistance to diseases in Poultry
Horizontal or General Resistance
The resistance of a host to all the races of a pathogen is called horizontal resistance. This type of resistance
is governed by number of genes or multiple alleles.
How Genetic Resistance is developed?
When changes in the genetic makeup are occurred then there will be diversity in the host as well as in the
pathogens and it is explained as follows:
“The number of individuals within a population that exhibit resistance to infection (bacterial, viral or other)”
Modes of Genetic Resistance
Genetic Modifications (GM)
Genetic resistance is only be achieved by making modifications in the genome of the host as well as the
disease producing organism. This includes:
1) Dominant Negative Protein
2) Ribonucleic acid interference
3) Ribonucleic acid decoys
4) Transgenic Production of Antibodies
5) Transgenic Birds
6) Pronuclear Injection
7) MHC
Dominant Negative Protein
The introduction of mutant versions of key factors in pathogen infection, such as cell surface receptors, can
block disease progression.

70
Ribonucleic acid interference
This strategy relies on the ability of specific short RNA sequences to anneal with the RNA of the pathogen,
causing destruction of the foreign RNA. RNAi requires access to the target RNA, which may limit this
approach to viruses.
Ribonucleic acid decoys
The expression of RNA sequences that mimic specific sequences within a pathogen can disrupt the activity
of the pathogen’s replication machinery. Again, this approach is probably restricted to specific viruses, with
influenza being a good candidate.
Transgenic Production of Antibodies
The transgenic production of antibodies in the host animal may act in an analogous manner to vaccination.
Transgenic Birds
Those birds which are genetically modified into well-defined form for a useful purposes by addition or
subtraction of their genetic material.
Techniques for the production of transgenic Birds
There are significant differences between the reproductive physiology of birds and mammals, which have
made it impossible to directly apply methods of genetic modification developed for mammals to the
production of transgenic birds. Avian embryos develop from a very large yolky egg that is enveloped in a
hard shell after fertilization and then incubated. By the time a fertile egg is laid, the chick embryo has
already developed to a stage at which the embryo consists of approximately 60,000 cells. Despite the
technical challenges, several methods are being developed that have been successful to some extent.
Zygotes recovered from laying hens shortly after fertilization can be injected with transgene constructs and
cultured to give hatched chicks, 1% of which will be transgenic. It is easy to obtain fertile new-laid eggs,
but as development of the embryo is advanced at the time of laying, attempts to genetically modify embryos
at this stage result in production of chimeric animals that may not have transgenic germline cells.

71
The greatest successes in the production of transgenic birds have been achieved using vectors derived from
avian retroviruses. Different retroviral vectors have been developed, with the highest success achieved by
using a vector derived from spleen necrosis virus, where one transgenic male is identified from 15 potential
transgenics. The use of avian virus vectors has technical problems associated with poor levels of expression
of transgenes carried by the vectors and potential high risks of recombination with retroviruses that are
prevalent in commercial poultry populations.
Pronuclear Injection
Direct introduction of a DNA construct into one of the two pronuclei of the fertilized egg, was the
technique used to produce the early transgenic livestock. Efficiency is only 3-5% (unclear).
MHC (Major Histocompatibility Complex)
It is a set of cell surface molecules encoded by a large gene family which controls a major part of the
immune system in all vertebrates. “In Humans it is called as human Leukocyte antigen (HLA)”
Functions of MHC
 To bind peptide fragments derived from pathogens and display them on the cell surface for
recognition.
 Also determines compatibility of donors for organ transplant, as well as one's susceptibility
to an autoimmune disease.
New methods
There have been numerous recent developments in animal transgenesis. Some – such as sperm-mediated
gene transfer especially if combined with intracytoplasmic-sperm injection are appealing, but still lack the
robust character needed to attract more general interest. In the opinion of the authors, the most encouraging
development with respect to the genetic modification of livestock relates to the use of viral vectors,
particularly those based on lentiviruses that offer a solution to current limitations through dramatic increases
in efficiency.

72
This use of this type of vector-delivery of transgenes may mark the beginning of a new era in transgenic
livestock simply by reducing the cost involved in their generation.
Importance of Genetic Resistance
 Selective breeding
 Improves the genetic make up for disease resistance
 Exploited low cost opportunity for disease control
 Environmental safety
The future for genetically modified farm animals
Diseases in farm animals can be controlled by vaccination, the use of drugs, improved husbandry and by
breeding animals for improved resistance. Successful management of disease is likely to include a
combination of approaches. There are some diseases which are modified according to their environment or
either they are going to be with different protocols in the field conditions. Some Viral as well as Bacterial
diseases are given in the tables as follows:
Agents Diseases Comments
Virus MD MHC resistance gene used in breeding programs
Virus ILT
Inbred lines differ in resistance
MHC genes involved
Virus AL Single dominant gene affects resistance
Virus IBD Breed differences in resistance
Virus IB Inbred lines differ in resistance
Virus ND Heritable differences in resistance long established
Bacteria Pullorum Inbred lines differ in resistance
Bacteria Fowl Typhoid Inbred lines differ in resistance
Bacteria Salmonellosis Well established resistance effects

Poultry Breeding and Production manual by Dr.Shamoil Tariq

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