Std 12 chapter 5

BIOLOGY
STD-12
PRINCIPLES OF
INHERITANCE & VARIATION
SANJAY SIDDHAPURA
M.SC.,B.ED., GSET, GPSC

Introduction
• Genetics : It is a branch of biology that deals with the study of heredity and variation.
• Heredity : It involves transmission of characters from parents to offspring.
• Clone : It is product of asexual reproduction. Clones are carbon copies of one
another derived from a common ancestor.
• Variations : It means the differences (Morphological, Physiological to logical and
behaviouristic) amongst the individuals of the same species and the offspring of the
same parents.

Terminology
• 1. Factor : Mendelian factor is a unit of inheritance, which passes from one generation to
the next through the gamete and controls the expression of a character in the organisms.
• In modern genetics, It is called ‘Gene’ which consists of DNA and genes are responsible
for the inheritance and expression of a character.
• 2. Allele or Allelomorph : Contrasting forms of a gene which are found on the same locus
in the two homologous chromosomes & control the expression of a trait are called alleles.
Ex : Tallness (T) and dwarfness (t).
• 3. Phenotype : It represents the expression of external appearance like colour, shape etc.
of an individual. Ex: Red colour, tallness or dwarfness etc.
• 4. Genotype : It indicates the genetic constitution of an individual. Ex: The genotype of
hybrid tall pea plants is Tt, pure tall TT and dwarf tt.
• * Johannson (1911) firstly used the term ‘Phenotype ‘and’ Genotype’.

• 5. Homozygous : It is an individual which contains identical alleles of a gene or
factor of a character on its homologous chromosome Ex: TT and tt
• 6. Heterozygous : It is an individual which contains the two contrasting factor of a
character or two different alleles of a gene on its homologous chromosomes. Ex : Tt
• 7. Hybrid : The organism produced after crossing two genetically different
individual is called hybrids.
• 8. Hybridization: The process of obtaining hybrids is called hybridization.
• 9. Reciprocal cross : If in one cross individual ‘A’ is used as male and ‘B’ as female
and in the next cross ‘B’ is used as male and ‘A’ as female, It is called as reciprocal
cross.
• 10. Homologous pair : Zygotic pair is called Homologous pair.

• 11. Back Cross : It is a cross between F1 hybrid and one of its parents. Back cross
includes test cross.
• 12.1 Test cross : It is a cross between F1 hybrid and recessive parent to know whether an
individual is homozygous or heterozygous for dominant character.
• 12.2 Out cross : If cross is performed between F1 hybrid and dominant parent then it is
called ‘Out cross’. All the offsprings obtain from this cross have dominant characters.
• 13. Genome : A complete set of chromosomes found in each nucleus of given species is
called genome. A single genome is present in haploid cell(n).
• 14. Gene Pool : Sum of all the genes and their alleles present in an interbreeding
population is called gene pool.
• 15. Phenocopy : When the different genotypes produce the same phenotypes due to
different environments, then one is called the phenocopy of the other Ex: All seeds
irrespective of their genotypes, germinating in dark develop yellow leaves.
Phenocopy is not inheritable. The term phenocopy was introduced by Goldschmidt in
1935.

• 16. Pure line : The term was coined by Johannson in 1900. It is a strain of
genetically pure true breeding individuals which have devised by a single self
fertilized homozyous ancestor or indentical homozygous ancestors.
• 17. Checker Board : It was firstly used by C. Punnett. The analysis of
generation is represented in the form of symbols of squares. Female gametes
arrange vertically and male gametes arrange horizontally.

Mendelism
• Gregor johann Mendel is known as ‘Father of Genetics’ Mendel proposed that inheritance is
controlled by paired germinal units or factor, Now called genes.
• Mendel performed hybridization experiment on garden pea (Pisum sativum) from 1856 to 1864. After
8 years of experimentation he read out the results of his observations of the Natural history society
of Brunn in 1865.
• ‘Nature for schender varien’ was a magazine of german language which was published by ‘Natural
history society of Brunn’.
• In this magazine, main research paper of Mendel “Versuche liber planzen Hybriden” or
Experiment in plant-hybridization” was published in 1866.
• Mendel’s work remained unnoticed for 34 years due to following regions
• (i) The scientific world was being rocked at that time by Darwin’s theory of evolution (Origin of
Species,1859)
• (ii) Limited circulation of the ‘Proceeding of Natural history society of Brunn’ in which it was
published.
• (iii) Mendel’s conclusion about heredity were ahead of his time. Mendel died in 1884 long before his
work came to be recognised.

Re-discovery of Mendel’s work
• It was in 1900 that three scientists independently rediscovered the principles
of heredity already worked out by Mendel.
• They were Eric von Tschermak of Austria, Hugo de Vries of Holland
and Carl correns of Germany.
• * W. Bateson (1906) firstly used the term ‘Genetics’.
• He is known as ‘father of modern genetics’.
• Muller established ‘Cytogenetics.

Reason’s for Mendel’s Success
• 1. Mendel firstly analyzed the work of former scientists and then prepared his strategy of
experiment.
• 2. Mendel took one or two characters at one time for his breeding experiment.
• 3. Mendel selected the pea plant for experiment because
• (i) Pea plant is Herbaceous and annual. It can be grown two or three time in a year. So
the results obtain very shortly.
• (ii) Pea’s flowers are bisexual and self pollinated. So Emasculation and cross pollination
can be applied in pea flowers.
• (iii) It can easily grow in garden.
• (iv) Many contrasting characters are found in pea plant. Mendel selected 7 pairs of
contrasting characters of pea plant. These characters are as follow.

Reason’s for Mendel’s Success
• 4. Mendel kept a complete record of every cross and used statistical method and law of
probability for analyzing his results.
• 5. He took care to avoid contamination from foreign pollen grains brought by insects.
• 6. Mendel selected only pure breeding varieties of pea for his experiment and grew in separate row.
• 7. Mendel studied the inheritance of character till F3 generation.

MONOHYBRID CROSS
• It is cross between two organisms of a species, that are
different in a pair of contrasting characters. Ex: height of
plant.
• Firstly Mendel selected long and dwarf plants of garden pea.
• Mendel removed stamens of the.flower of tall plants in bud
condition. It is called ‘Emasculation’.
• A Bag tied over the flower for the prevention of cross-
pollination. This is called ‘Bagging’.
• Dwarf plant took as male plant and their flower covered
through bag.
• On dehiscence or maturity, the pollen grains of dwarf plant
sprayed over the stigma of long plant and It again covered
through bag.

• Seeds collected through long plant. After ward Mendel obtained F1 generation through the sowing
of former.
• This process called ‘Hybridization’ and F1 generation called ‘Hybrid’.
• Mendel obtained only tall plants in F1 generation.
• Mendel applied reciprocal cross but the results did not affect.
• After this process, Mendel obtained F2 generation through the sowing of those seeds which
obtained from self pollination of F1 plants.
• In F2 generation, Tall and dwarf plants obtained in 3 : 1 ratio. It is prooved that the character of
tallness in the plants of F1 generation does not pure.
• Mendel used this technique for other six characters in their experiment and obtained same results.
• The phenotypic ratio of F2 generation in Monohybrid cross is 3 : 1 and genotypic ratio is
1 : 2 : 1.
• The plant possesses two factors of each character. The term ‘Factor’ was fIrstly used by Correns
Which was called determinor or element by Mendel.

CONCLUSION OF MONOHYBRID CROSS
• (I) Principle of paired Factors : A plant possesses
two factors of each character. Each trait is controlled
by a unit factor.
• (II) Principle of Dominance : Out of the two
factors or alleles representing the alternate form of a
trait, one is dominant and expresses it self in the
hybrid of F1 generation. The other factor or allele is
recessive and does not show its effect. It is called the
‘Mendel’s law of dominance’. The factor of
dominant character is denoted by capital letter and the
factor of recessive character is denoted by small letter.

Exceptions of Principle of Dominance and
Paired Factors
•1. Incomplete dominance
•2. Co-dominance
•3. Multiple allele
•4. Pleiotropy

1. Incomplete dominance
• Incomplete dominance was discovered by Carl correns (1903) in Mirabilis jalapa.
• Incomplete dominance is the phenomenon where none of the two contrasting factor or
alleles is dominant.
• The expression of the character in a hybrid of F1 generation is intermediate (partial or
mosaic).
• Mirabilis jalapa is also known as 4’0' clock plant or Gulbansi
• In Mirabilis jalapa, there are two types of flower colour in pure state, Red and White.
• When the two types of plant are crossed. The hybrid of F1 generation have pink flower.
• If F1 generation are selfcross, the plants of F2 generation are of three types- Red, Pink
and White flowered in the ratio of 1: 2: 1
• The phenotypic and genotypic ratio is similar (1 : 2 : 1) due to incomplete
dominance in F2 generation.

• Incomplete dominance is also found
in colour of flower in Snapdragon
or Antirrhinum or Dog flower and
• Feather colour in Andulasian
fowls.

2. Co-dominance
• In codominance the alleles are able to express themselves independently in F1
generation.Ex : (i) Skin colour in Short Horned cattles.
• There are two types of pure cattle, Red and White. On cross-breeding the individuals of
F1 generation are found to have roan colour.
• Here in a heterozygote of F1 generation both alleles express themselves independently.
• The effect is produce due to juxtaposition of small patches of Red and White colour. It is
also known as mosaic inheritance.
• Both the alleles are co-dominant in F1 generation. On inbreeding the roan hybrids
produce three types of cattle-Red Roan and White in the ratio of 1:2:1.

• Ex: (ii) Sickle cell Haemoglobin. The allele for sickle
cell haemoglobin HbS is codominant with allele for
normal haemoglobin HbA.
• Ex: (iii) AB Blood Group. Alleles of blood group A(IA)
and blood group B (IB) are codominant & found over
the surface of erythrocytes. when they come together
in an individual, they produce blood group AB.
• Ex: (iv) MN Blood Group. It is found in humans. The
erythrocytes carry two types of native antigens, Mand
N, and an individual can be MM, MN, NN.
Example of Co-dominance

3. Multiple allele
• Bernstein (1924) : Discovered multiple alleles.
• More than 2 alternative forms of a gene are called as multiple allele.
• Multiple alleles are located on same locus of homologous chromosome.
• A diploid individual contains two alleles and gamete contains one allele for a character. Ex:
(i) Human Blood group - 3 alleles.
• (ii) Four alleles regulate coat colour in rabbit

4. Pleiotropy
• A gene regulates multiple phenotypic effect. Ex: sickle cell anaemia.

(III) Principle or law of segregation
• There is mixing of two factor in the hybrid of F1 generation.
• At the time of gamete formation in F1 generation, The two factor separate or segregtate
and pass into different gametes randomly.
• A gamete comes to have one factor of a pair.
• The gamete fuse randomly during fertilization so that the factors come together in F2
generation and express themselves freely.
• So the dwarf plants obtain in F2 generation.
• Gametes are always pure for a character hence it is also called ‘law of purity of
gametes’. (by Bateson)

Dihybrid cross
• It is a cross between two organisms of a species that are different in two pairs of
contrasting characters.
• Mendel selected the following two character for this purpose.
• (i) Colour of cotyledon - Yellow and green
• (ii) Shape of seed - Round and wrinkled
• In which Yellow and Round traits are respectively dominant over green and wrinkle traits.
• Mendel performed cross between pure breeding pea plants having yellow round seeds
(YYRR) and pure breeding Pea plants having green ,wrinkled seeds (yyrr).
• All the plants of the F1 generation were yellow and round seeds (YyRr).
• The factors of both characters will have independently segregated to each other during
gamete formation.
• Thus total four types of gametes(YR), (yR), (Yr), (yr) form in F1 generation
• On selfing of F1 the resultant F2 generation show four types of plants

• Thus the Conclusion of dihybrid cross
• Number of genotype in any cross is = 3n
• Number of genotypes in dihybrid cross = 32 = 9
• Yellow round and green wrinkled plants show parental combinations (9+1) while green
round and yeIIow wrinkled show new combinations (3 + 3 = 6). The ratio between parental
combination and new combination is 5 : 3.
• So 62.5%Parental combination and 37.5% new combination are obtained.
• Law of Independent assortment :
• Mendel concluded that the two factors of a character assort or separate independent of the
factors of other characters at the time of gamete formation and get randomly rearranged in
the offsprings. It is called the ‘Law of Independent assortment’.
• Objection :
• This law is applicable to only those factors or genes which are either situated distantly on the
same chromosome or occur on different chromosomes.

Back cross
• It is a cross which is performed between F1 hybrid and one of its parents.
• (a) Out cross (b) Test cross
• (a) Out cross
• If cross is performed between F1 hybrid and dominant parent then it is called ‘Out
cross’.
• All the offsprings obtain from this cross have dominant characters. Thus F1 generation
can be analized through this cross.
• (b) Test cross
• Cross is performed between F1 hybrid and recessive parent. It is a cross to know whether
an individual is homozygous or heterozygous for dominant character.

Importance of Mendelism
• 1. It is useful in the field of plant breeding. Advanced features can be used in the
development of improved varieties of offspring through the hybridization method.
• 2. The knowledge of dominant and recessive characters is possible through mendelism.
• 3. Development of high productive varieties and disease resistance varieties is possible.
• 4. It is also useful for the development of improved varieties in animals.

Post mendelian inheritance
• It takes place between two alleles of a gene
Gene Interaction
1. Intragenic 2. Intergenic
Incomplete
dominance
Multiple
alleles
Codominance
(i) Epistasis
(ii) Complementary Genes
(iii) Supplementary Genes
(iv) Duplicate Genes
(v) Polymeric or Additive Genes
(vi) Collaborative Supplementary
Genes
(vii) Qualitative and Quantitative Inheritance
(viii) Lethal gene
(a) Dominant epistasis
(b) Recessive epistasis
(c) Dominant Recessive
Epistasis

2. Intergenic
• It is nonallelic interaction in which two or more independent genes present on same or
different chromosomes interact to produce a different expression.
• (i) Epistasis
• In this type of interaction one gene suppresses the expression of another nonallelic gene. The former
is called epistatic gene whereas the latter is called hypostatic gene.
• Epistasis comprises three types.
• (a) Dominant epistasis
• In this type dominant gene of one locus suppresses the expression of another gene of different locus.
• Ex: Fruit Colour in Cucurbita pepo (Summer squash)-
• A cross between a pure breeding white summer squash (WWYY) with a pure breeding green summer
squash wwyy produce white fruits in the F1 generation.
• Dominant gene W is epistatic over Y & w & y genes so that phenotypic ratio of F2 generation comes to
have 12 white fruit, 3 yellow fruit, 1 green fruit.

• (b) Recessive epistasis : In this type recessive homozygous genes of one locus
suppresses the expression of another nonallelic gene Ex: Coat Colour in Mice.

• (c) Dominant Recessive Epistasis :
• Here the dominant allele at one locus (I) and recessive allele at another locus (cc) give rise to
the same effect (I - C, I - cc, iicc). Ex: Plumage Colour in Poultry

(ii) Complementary Genes
• In this type, both nonallelic genes independently produce similar effect when they
come together in the dominant form they form a new trait.
• Ex: Flower colour of Sweet Pea (Lathyrus odoratus) - Flower colour is purple
if dominant alleles of two nonallelic genes (C – P –) are present together . The
colour of flower is white if the double dominant condition is absent (ccP –, C–
pp, ccpp).
• Purple colour formation is two step reaction and the two genes cooperate to form
the ultimate product.

(iii) Supplementary Genes
• In this type dominant allele of one gene produces its effect independently where as dominant
allele of the second gene is without any independent phenotypic effect but is able to modify
the expression of the first gene & produce a new trait. Ex: seed coat colour in Lablab.

(iv) Duplicate Genes
• Two or more independent genes lie on different chromosomes produce same phenotype
therefore either them can form the same phenotype in homozygous or heterozygous state.
Independent genes do not form cumulative effect. Ex: Fruit shape in Shepherds’s Purse
(Capsella bursa).

• (vi) Collaborative Supplementary Genes :
• Two nonallelic genes produce their own phenotypic effect independently when present in
the dominant state but can also interact to form a new trait. Ex: Comb types in poultry.
P gene independently produces Pea comb. R gene
independently produces rose comb.Both P and R
jointly form walnut comb. When none of these genes
is present in the dominant state (pprr), single comb is
formed.

WHY MENDEL’S WORK REMAINED UNRECOGNIZED ?
• Mendel published his work on inheritance of characters in 1865 but for several reasons, it
remained unrecognised till 1900.
• (1) Communication was not easy (as it is now) in those days and his work could not be
widely publicised.
• (2) His concept of genes (or factors, in Mendel’s words) as stable and discrete units that
controlled the expression of traits and, of the pair of alleles which did not ‘blend’ with
each other, was not accepted by his contemporaries as an explanation for the apparently
continuous variation seen in nature.
• (3) Mendel’s approach of using mathematics to explain biological phenomena was totally
new and unacceptable to many of the biologists of his time.
• (4) Mendel’s work suggested that factors (genes) were discrete units, he could not provide
any physical proof for the existence of factors or say what they were made of.

CHROMOSOME BASIS OF INHERITANCE
• In 1900, three Scientists (de Vries, Correns and von Tschermak) independently rediscovered
Mendel’s results on the inheritance of characters.
• Also, by this time due to advancements in microscopy that were taking place, scientists were able
to carefully observe cell division.
• This led to the discovery of structures in the nucleus that appeared to double and divide just
before each cell division. These were called chromosomes (colored bodies, as they were visualised by
staining).
• By 1902, the chromosome movement during meiosis had been worked out.
• Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the
behaviour of genes and used chromosome movement to explain Mendel’s law.

CHROMOSOME BASIS OF INHERITANCE
• It was proposed by Sutton and Boveri independently in 1902 and expanded by Morgan, Sturtevant and
Bridges. Main features of chromosome theory of inheritance are as follows
• (1) Sperm and ovum are bridge between two generations.
• (2) Both sperm and ovum contribute equally in heredity of offspring the sperm provides only nuclear
part to the zygote.
• (3) Nucleus contains chromosomes therefore, chromosomes carry the hereditary characters.
• (4) Each chromosome or chromosome pair plays definite role in the development of an individual. Loss
of a part / complete chromosome causes structural and functional deficiency in the organism.
• (5) Both chromosomes appear in pair in the somatic or diploid cells.
• (6) A gamete has only one chromosome as well as one mendelian factor out of homologous pair .
• (7) Synapsis and random independent separation of chromosomes form the quantitative basis of
independent assortment of mendelian factors.
• (8) Paired condition of chromosomes is retained during fertilization.
• (9) Sex of certain organisms is determined by specific chromosomes called sex chromosomes.

Sex Determination
• On the basis of fertilization, Sex Determination is of three types.
• (i) Progamic : Sex is determined before fertlization Ex: Drone in honey bee.
• (ii) Syngamic : Determination of sex takes place during fertilization Ex: most of the
animals and plants
• (iii) Epigamic : Sex determination takes place after fertilization Ex: Female in honey
bee.

Types of sex determination
• (A) Allosomic determination of sex : Chromosomes are of two types .
• 1. Autosomes or somatic chromosomes :
• They control inheritance of somatic characters.
• 2. Sex chromosomes or Allosomes or Heterosomes :
• In most of the animals and some plants, sex is determined by sex chromosomes or allosomes.
• X-chromosome or X-body discovered by Henking in the testes of male bug. Sex
chromosome
• discovered by “Mc Clung” in Grasshopper.
• Stevens discovered Y-chromosome.
• Wilson and stevens (1905) proposed chromosomal theory of sex determination.

Types of Allosomic determination of sex
• (i) XX—XY type or Lygaeus type
(XX - XY type) Sex determination in plants
• .
• .
• .
• .

(i) XX—XY type or Lygaeus type
• It was firstly described by Wilson & Stevens in Lygaeus insect.
• In this types female is homogametic and forms only one type of gamete.
• Male is heterogameticand produces two types of gametes.
• Ex: human, Drosophila, dioecious plant like Coccinea,
Melandrium.

Sex determination in plants (XX - XY type)
• Amongst angiosperms XY sex determiantion is found in Melandrium (Lychnis),
Coccinia, salix, Elodea.
• Sex determination in Melandrium album (Caryophyllaceae) studied by Warmke,
(1946) and Westerguard, (1958).
• Here X - chromosome is shorter then the Y- chromosome.
• The Y chromosome bears four regions.
• The X-chromosome contains two regions.
• Pro. R. P. Roy discovered sex determination in Coccinea indica & gave the importance
of Y- chromosome in plants.

• 1. Female suppressor region : It
suppresses the formation of female
reproductive organs.
• 2. Male promoter region : It starts
development of Anther.
• 3. Male fertility region : It
proceeds further development of
Anther.
• 4. Homologous region : It
initiates disjunction & Pairing of X
and Y chromosome during meiosis.
• 5. Differential region of X-
chromosome : It promotes the
development of female sex organs.
STRUCTURE OF PLANT SEX CHROMOSOME

• Female is heterogametic and produces two types of ova while male is homogametic and
produces one type of sperms.
• Ex: Birds, Reptiles, Some fishes, Hen, Plant Fragaria eletier.

• Female is homogametic and male is heterogametic.
• Actually male bears only one sex chromosome therefore male produces two types of
sperm A + X and A+ O . whereas female produces only one type of egg.
• Ex: Squash bug - Anasa, Grass hopper, Cockroach, Ascaris and plants like
Dioscorea sinuta & Vallisneria spiralis.

(v) Haplo-diploidy type
• In animals such as honey bee female is diploid and male (drone) is haploid.
• Drone or male insects are develop by unfertilized eggs through parthenogenesis.
• It is called arrhenotoky.
• Meiosis does not occur during the formation of sperms.
• Females develop from fertilized eggs and are hence diploid.
• Queen Bee takes all the sperms from the drone during nuptial flight and stores them in saminal
vesicle.
• They do not fertilise the eggs & develop into males or drones.

Hormonal theory for sex determination
• In some cattles cow, sheep, goat, dizygotic twins are common in which one foetus
may be male and the other female.
• Male foetus secretes male hormone before secretion of female hormones that
suppresses the development of ovaries in the female foetus, it is called Free martin
(Twins) in which male is fertile and female is sterile.

Gynandromorphs
• Some Drosophila individuals were
found to possess half of the body
of male and half of female they
are called Gynandromorphs.
• The latter is formed by loss of
one X-chromosome at metaphase
plate during first zygotic division.

Genic balance theory
• It was proposed by Calvin Bridges (1926). He stated that sex is determined by the genic
balance or ratio between X-chromosomes and autosomes.
• It is applicable for Drosophila melanogaster in which expression of maleness is not
controlled by Y-chromosome.

(B) Environmental sex determination
• In some animals sex is influenced by environmental factors for example in Crepidula
(Molluscs), larva gives rise to female organism in isolation. If it is connected with female it
gives rise to male similar concept has been observed in Bonellia.
• In Reptiles sex determination depends on the temperature of incubation of eggs. In Agama
agama lizard female offspring produce through those eggs that are incubated on 26 – 27ºC
but they produce male offspring if they are incubated on 29ºC.
• In contrast - Tortoise (Graptemys) - low temperature (Less than 30ºC) promotes formation
of male & higher temperature (More than 30ºC) promotes female tortoise.
• In crocodile male offsprings produce on 33ºC whereas female offsprings produce on 31ºC.

(C) Nonallosomic
• In Bacteria sex determination is controlled by fertility factor located on a plasmid.
• In several simple organisms, sex is determined by single gene. Ex: Chlamydomonas,
two genes in Maize - One gene for male inflorescence and second gene for female
inflorescence .

Linkage
• Sutten and Bovari firstly stated about linkage in chromosomal theory of inheritance.
• Bateson and Punnet worked on sweet pea (Lathyrus odoratus) and proposed
coupling and repulsion hypothesis .
• They found that the factors for certain characters do not show independent assortment
thus linkage is exception of independent assortment.
• Therefore the dihybrid ratio is only 3 : 1 and a test cross ratio of 1 : 1.
• According to Morgan (Father of experimental genetics) :The genes of a chromosome
have tendency to inherit together and maintains the parental combination in successive
generations these genes are called linked genes and this phenomenon is called Linkage.

CHROMOSOMAL THEORY OF
LINKAGE
• Morgan & Casttle (1911) proposed chromosomal theory of linkage
• (i) Linked genes appear in the same chromosome.
• (ii) Genes arrange in Linear fashion on chromosome.
• (iii) The strength of linkage is inversely proportional to the distance between two genes.
• (iv) Genes present on same chromosomes have tendency to maintain parental
combinations except for occasional crossovers.

Types of linkage
• (1) Complete Linkage (2) Incomplete Linkage
• (1) Complete Linkage
• The gene situated on the same chromosome do not separate and are inherited together over
the generations due to the absence of crossing over. It is rare Ex : Male Drosophila.

(2) Incomplete Linkage
• The linked genes do not always stay together because homologous non sister chromatids
may exchange segments of varying length with one another during meiosis. It is called
crossing over.
• The linked genes located in a chromosome have chances of separation by crossing over,
are called incompletely linked genes and the phenomenon of their inheritance is called
incomplete linkage. Ex: Morgan conducted two crosses in Drosophila.
• Morgan attributed this due to the physical association or linkage of the two genes and
coined the term linkage to describe this physical association of genes on a chromosome
and the term recombination to describe the generation of non-parental gene
combinations.

• (i) Crossing of yellow bodied
(y) and white eyed (w) female
with brown bodied (Y+) red
eyed (W+) male produced F1
to be brown bodied red eyed.
After intercrossing of F1
generation.
• Morgan found that F2 ratio
deviated significantly from
expected 9 : 3 : 3 : 1 ratio. He
observed 98.7% parental types
and 1.3% recombinants.

• (ii) Second cross performed between
white eyed (w w) and miniature
winged (m m) female with wild type
or red eyed (w+ w +) normal winged
(m+ m+) males. In F1 generation
wild types form.
• Now F1 female fly test crossed with
white eyed and miniature winged
male. He observed 62.8% parental
types and 37. 2% recombinants in
offsprings.
• Note: The strength of linkage
between y and w is higher than w and
m.

Sex Linkage or Sex-Linked Inheritance
• Inheritance of some traits is controlled by genes of sex chromosomes.These are called
sex linked traits.
• It was discoverd by Morgan.
• (i) Criss Cross Inheritance :
• it is a type of sex linked inheritance where a parent passes the traits to the grand child
of the same sex through offspring of the opposite sex.
• It is of two types
• (a) Diagynic ( Diagenic) : Father to grandsons through daughter.
• (b) Diandric : Mother to grand daughter through son.
• (ii) Non Criss Cross Inheritance : It involves two types.
• (a) Holandric : Direct from father to son or male to male
• (b) Hologynic : Direct from mother to daughter or female to female.

Types of Sex Linked
Inheritance
(i) Criss Cross
Inheritance
(a) Diagynic
( Diagenic)
(b) Diandric
(ii) Non Criss Cross
Inheritance
(a) Holandric (b) Hologynic

(1) Sex Limited Traits
• These traits are expressed in a particular sex although their genes also occur
in the other sex they require sex hormones for their expression.
• Ex: moustaches and beards human males, breast in human females,
milk secretion in human females.

(2) Sex Influenced Traits
• These traits are controlled by autosomeal genes & the expression of these genes in a
particular sex depends upon favourable conditions.
• Example. The gene of pattern baldness acts as a dominant one in human males because
it can express itself due to availability of male hormones. It is unable to express in the
human females due to absence of hormones.

MUTATION
• Sudden inheritable change in an organism is called mutation.
• Darwin coined the term sports and Bateson coined the term saltation or discontinuous variations for
them.
• Mutation was discovered by Hugo de vries in Oenothera lamarckiana (Evening primerose). He
observed 834 mutations in 54343 plants of Oenothera.
• Later workers found that ‘mutations’ observed by De Vries were actually chromosome aberration and
polyploids.
• Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in
changes in the genotype and the phenotype of an organism.
• In addition to recombination, mutation is another phenomenon that leads to variation in DNA.

• On the basis of cytology mutation involves following types.
• (A) Chromosomal Mutation
• (B) Genomatic Mutation
• (C) Gene Mutation
• (A) Chromosomal mutation or chromosomal aberration
• There are two types of chromosomal mutation.
• (i) Structural changes of chromosomes
• (ii) Numerical changes of chromosomes.
• (i) Structural changes of chromosomes
• (a) Intra chromosomal type:- (1) Deletion (2) Inversion
• (b) Inter chromosomal type:- (1) Duplication (2) Translocation(3) Insertation
• Chromosomal aberrations are commonly observed in cancer cells.

(B) Genomatic Mutation
• Changes in chromosomal Number. It involves
• (1) aneuploidy and (2) Euploid (polyploidy)
• (1) Aneuploidy : In which one or few chromosomes are either deficient or in excess in a
species. It is of two types.
• (a) Hypoploidy : In which either one or few chromosomes are deficient. It involves two types.
• (i) Monosomic (2 n – 1) : In which one chromosome is deficient. Ex: Turner’s syndrome (44 + X 0), It is also
found in Gossypium.
• (ii) Nullisomic (2n – 2) : A pair of chromosome is deficient.
• (b) Hyperploidy : In which one or few chromosomes are in excess. It is of two types.
• (i) Trisomy (2n + 1) : One chromosome is additional in a pair of chromosome. it represents as 2n + 1. Ex: Datura,
• In human Trisomy in 21st pair called down’s syndrome, Trisomy in 18th pair called Edward syndrome and Trisomy in
13th pair are called. patau syndrome. Double trisomic has two different chromosomes in triplicate (2n + 1 + 1).
• (ii) Tetrasomic (2n + 2) : A pair of chromosome is additional or a chromosome is found fourtimes
Ex: Super female 44 + XXXX, Double tetrasomic (2n + 2 + 2).
• (iii) Pentasomic (2n + 3) : A chromosome occurs five time Ex: superfemale (44 + XXXXX) rare.

(2) Euploid (polyploidy)
• If an organism has more than two sets of chromosomes, It is called Polyploid this
phenomenon is called polyploidy.
• On the basis of occurence of number of genome, polyploid is called triploid (3n),
tetraploid (4n), Pentaplaoid (5n), Hexaploid (6n) etc.
• In nature, polyploidy appears due to the failure of chromosomes to separate at the time of
anaphase either due to nondisuction or due to no formation of spindle.
• Polyploids with odd number of genomes (Ex: triploids, pentaploids) are sexually sterile.
so that they perform reproduction by vegetative propagation Ex: Banana, Pineapple.

• (2) Euploid (Polyploidy) involves three types :
• (i) Autopolyploidy (ii) Allopolyploidy (iii) Autoallopolyploidy
• (i) Autopolyploidy :
• It is a numerical increase of the same genome such as Autotriploidy (AAA) Ex: Rice, Gram,
Maize.
• (ii) Allopolyploidy :
• It is formed by hybridisation between two species followed by doubling of chromosomes such as
(AABB). Ex: Wheat, Tobacco, artificially produced two allopolyploids are Raphano
brassica and Triticale.
• (iii) Autoallopolyploidy :
• One genome is in more than diploid state such as (AAAABB) Ex: Helianthus tuberosus.
• Polyploidy can be artificially induced by application of colchicine and granosan.

(C) Gene Mutations
• Sudden stable changes in the structure of gene or cistron due to change in nucleotide
sequence and nucleotide type are called gene Mutations.
• Most of the gene mutations include change in single nucleotide. These are called point
mutations.
• If mutations takes place in more than one base pair is called gross mutation.
• Usually Gene mutations appear during replication of DNA, therefore It is called copy
error mutation.
• Seth Wright (1791) firstly recorded point mutation. He observed short legged lamb
(ancon sheep).
• Scientifically Thomas hunt Morgan observed actual mutation (gene mutation) when he
found white eyed mutant of male Drosophila among wild type red eyed Drosophila.

•Type of gene mutations :
• Gene mutations involve three types
• (i) Substitution:It includes replacement of one type of nitrogenonus base by other. It is of two types
• (a) Transition : In this type, one purine is replaced by another purine while one pyrimidine by
another pyrimidine.
• (b) Transversion : Purine base is replaced by a pyrimidine base or vice versa.
• (ii) Inversion : A deterioration of DNA by mutagen can change the base sequence of a cistron in the
reverse order.
• (iii) Frame-shift Mutations : In this types the entire reading frame of base sequence shifts laterally either
in the forward direction due to insertion of one or more nucleotides or in
the backward direction due to deletion of one or more nucleotides. It is
also called gibberish mutations. It is of two types
• (a) Deletion : One more nucleotides are eliminated from segment of DNA or gene.
• (b)Insertion : Addition of one or more nucleotides in the segment of DNA or gene.

MUTAGENS
• Any type of physical or chemical factor that is used in the induction of mutation are called
mutagens. The mutagens are of two types.
• (i) Physical mutagens : These are of two types
• (a) High Energy Radiations
• (1) Ionizing radiations : Ex: X-rays, gamma rays and cosmic rays , alpha particles, beta rays
• (2) Non-inoizing radiations : Ex: ultra-violet light.
• (ii) Chemical Mutagens
• (a) Base Analogues: Ex. 5-bromouracil
• (b) Alkylating Agents : Nitrogen mustards
• (c) Nitrous Acid
• (d) Acridines: Ex: acriflavine, proflavine
• Deletion or additionof nucleotides cause frame shift mutation.

Human Genetics
• Archibald Garrod, firsty studied human genetics.
• He studied black urine disease alkeptonuria and stated that it is inherited by recessive gene.
• This was also the first study of inborn error of metabolism and biochemical genetics,
hence Garrod is also known as “Father of human genetics” or “Father of biochemical
genetics” .
• Important techniques used to study human genetics are.
• 1. Pedigree Analysis
• 2. Study of Twins
• 3. Population Genetics
• 4. Karyotype
• 5. Cell Culture

Pedigree Analysis
• A record of inheritance of certain genetic traits for two or more generations presented in
the form a diagram or family tree is called pedigree. Ex: Human, domesticated animals
• Square represents the male . A circle represents the female .
• Solid symbol shows the trait under study and a cross or shade of any type or , or
Words can also be used in place of symbols.
• Parents are shown by horizontal line while their offsprings are connected to it by a vertical
line.
• Pedigree analysis is study of pedigree for the transmission of particular trait and finding
the possibility of absence or presence of that trait in homozygous or heterozygous state
in a particular individual.
• It is useful for the genetic counsellors to advice intending couples about the possibility of
having children with genetic defects like haemophilia, colourblindness, alkaptonuria,
phenylketonuria, thalassemia, sickle all anaemia, polydactyly & syndactyly.

• Each and every feature in any organism is controlled by one or the other gene located on
the DNA present in the chromosome.
• DNA is the carrier of genetic information.
• It is hence transmitted from one generation to the other without any change or alteration.
• Changes or alteration do take place occasionally.
• Such an alteration or change in the genetic material is referred to as mutation.
• A number of disorders in human beings have been found to be associated with the
inheritance of changed or altered genes or chromosomes.
• Disorders caused by altered genes or chromosomes known as Genetic disorders .
• Genetic disorders may be grouped into two categories – Mendelian disorders and
Chromosomal disorders.

Genetic disorders
Mendelian disorders
Dominant Recessive
Chromosomal disorders
AUTOSOMAL
SEX
CROMOSOME
AUTOSOMAL
Ex. Myotonic dystrophy
SEX CROMOSOME
AUTOSOMAL
Ex. Sickle-cell anaemia,
Phenylketonuria
SEX CROMOSOME
Ex. Haemophilia
Down’s Syndrome
Klinefelter’s
Syndrome
Turner’s
Syndrome

Mendelian disorders
• Mendelian disorders are mainly determined by alteration or mutation in the single gene.
• These disorders are transmitted to the offspring on the same lines as we have studied in the
principle of inheritance.
• The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree
analysis.
• Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell
anaemia, Colour blindness, Phenylketonuria, Thalesemia, etc.
• Mendelian disorders may be dominant or recessive.
• By pedigree analysis one can easily understand whether the trait is dominant or recessive.
• Similarly, the trait may also be linked to the sex chromosome as in case of haemophilia.
• It is evident that this X-linked recessive trait shows transmission from carrier female to male
progeny.

(a) Autosomal dominant trait
(example: Myotonic dystrophy)
(b) Autosomal recessive trait
(example: Sickle-cell anaemia)

Haemophilia
• It is Sex linked disease (recessive gene of Haemophilia lies on X- chromosome) in which the
patient will continue to bleed even from minor cut .
• Queen Victoria also infected this disease. hence it is called royal disease. It is of two types.
• (i) Haemophilia – A : In this type patient does not possess the natural phenomenon of
blood clotting due to absence of factor VIII (AHG or Anti Hemophilic Globulin ).
• (ii) Haemophilia – B : It is due to lack of factor IX (Plasma thromboplastin factor).
• It shows its transmission from unaffected carrier female to some of the male progeny.
• In this disease, a single protein that is a part of the cascade of proteins involved in the
clotting of blood is affected.
• The heterozygous female (carrier) for haemophilia may transmit the disease to sons.
• The possibility of a female becoming a haemophilic is extremely rare because mother of
such a female has to be at least carrier and the father should be haemophilic (unviable in the
later stage of life).

• It is found in human male only because single gene for the Heamophilia is able to express
itself due to presence of one X- chromosome only.
• This defect does not appear in the girl babies because of the presence of the allele for the
normal blood clotting present on the second X -chromosome (XXh).
• Therefore, the girl babies remain carrier. A female becomes haemophiliac only when both
its X-chromosomes carry the gene (Xh Xh).
• However, such females normally die before birth because the combination of these two
recessive alleles is lethal.

CONDITION -1: FEMALE CARRIER & NORMAL, MALE NORMAL

Condition – 2: Carrier female, Haemophilic male

Condition – 3: Normal female, Haemophilic male

• Y- linked gene : The genes present on diffferential parts of Y-chromosome are passed
directly from father to son. These genes are called holandric genes.
• Ex: hypertrichosis (excessive hairs on ear pinna),
• TDF (testis determining factor),
• Keratoderma dissipatum (thickened skin of extremities),
• Porcupine skin,
• Webbed toes in humans,
• Masulatures (mac trait) in male
• Guppy fish (Lebistes).

Sickle-cell anaemia
• It occurs due to autosomal codominant (formerly considered recessive) allele HbS present on
chromosome 11.
• It is caused by the formation of an abnormal haemoglobin-S. In which Glutamic acid, of 6th
position in -chain is replaced by Valine amino acid due to substitution of T by A in the second
position of the triplet codon (CTC) on chromosome 11.
• The codon CTC usually transcribed into GAG (coding for Glutamic acid). but due to
substitutions of T by A The new codon CAC is transcribed into GUG that codes Valine.
• It is the major effect of allele.
• Other secondary effects involve formation of sickle-shaped erythrocytes during oxygen
deficiency. The cells cannot pass through narrow capillaries resulting the latter become clogged.
• Spleen and brain get damaged.
• The homozygotes having only haemoglobin-S (HbSHbS) usually die before reaching maturity due
to erythrocyte distortion but HbAHbS individuals survive.

Phenylketonuria
• It described by Folling (1934).
• It is an inborn error of autosomal recessive metabolic disorder.
• Enzyme phenylalanine hydroxylase is absent due to abnormal autosomal recessive gene
on chromosome 12 .
• Phenylalanine (amino acid) is not converted into tyrosine (amino acid) in liver.
• It is called hyperphenyl alaninemia.
• Accumulation and excretion of phenylalanine, phenylpyruvic acid and related compounds
through urine because of its poor absorption by kidney.
• Symptoms are mental retardation (IQ less than 20),
• Decreased pigmentation of hair and skin and eczema.
Phenylalanine
phenylalanine hydroxylase
TyrosineIn liver of normal Human

Chromosomal disorders
• The chromosomal disorders are caused due to absence or excess or abnormal arrangement
of one or more chromosomes
• Failure of segregation of chromatids during cell division cycle results in the gain or loss of
a chromosome(s), called aneuploidy.
• Failure of cytokinesis after telophase stage of cell division results in an increase in a whole
set of chromosomes in an organism and this phenomenon is known as polyploidy. This
condition is often seen in plants.
• Down’s syndrome results in the gain of extra copy of chromosome 21.
• Turner’s syndrome results due to loss of an X chromosome in human females.
• Klinefelter’s Syndrome results due to the presence of an additional copy of X-
chromosome resulting into a karyotype of 47, XXY

(1) Down’s Syndrome (Mongolian Idiocy)
• It was discovered by Langdon Down.
• In this Syndrome 21-pair of chromosomes
show trisomy (presence of an extra chromosome
number 21.
• It is caused by nondisjunction of 21st
chromosome pair during anaphase. Thus the total
number of chromosomes are 47 instead of 46 &
genotype is 45 + XY in male & 45 + XX in
female.
• Symptoms of this syndrome are broad fore-head,
rounded face, permanently open mouth with
protruding tongue, projecting lower lip,
Mongolian type eye lid fold,verticalfold
epicanthus on either side of nose, stubby
fingers,broad palm with characteristic palmer
crease, Short neck , little intelligence,
undeveloped Gonads and genitalia.

(2) Klinefelter’s Syndrome
• It is found in man. Its patient contains
47 chromosomes and Its genotype is
44 + XXY.
• These persons are sterile males having
undeveloped testes. sparse body hair,
mental retardation, and some female
characters like development of
breasts (gynaecomastia).
• It is due to the presence of additional
X-chromosome. It has one barr body.
One in every 500 male births is victim
of this syndrome.

(3) Turner’s Syndrome
• It is found in woman. The
patient has 45 chromosomes
instead of 46. and its
genotype is 44 + X 0.
• These are sterile females
having rudimentary ovaries,
small uterus undeveloped
breasts, short stature,
abnormal intelligence, webbed
neck. menstrual cycle is
abnormal or absent.
• One in every 3000 children is
a victim of this syndrome.

Supermales
(Jacob’s syndrome or criminal syndrome)
• The genotype of supermale is
44 + XYY & total number of
chromosomes are 47 instead of
46. These males are
characterised by abnormal
height, mental retardation and
criminal bent of mind (Jacob’s
syndromes) supermales are
more aggressive than normal
males due to over secretion of
male sex hormones.

Super female
• The genotype of superfemale is either 44 + XXX (47 chromosomes) or 44 + XXXX (48
chromosomes) or 44 + XXXXX (49 chromosomes). They have abnormal sexual
development and mental retardation.

Std 12 chapter 5

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