10. patterns of inheritance

Genetics
• Study of the patterns of inheritance
• Mendelian Genetics
– Gregor Mendel
– Pea plant experiments
• Grow easily
• Distinguishable characteristics
– Round/Wrinkly, Yellow/Green, Tall/Short
• Can control mating

Terminology
• Allele—Alternate forms of a gene
• Dominant Allele—trait that exerts its effects
whenever present
– Symbolized by capitol letter (B)
– Ex. Dwarfism, Huntington’s disease
• Recessive Allele—trait that is masked if a
dominant trait is present
– Symbolized by lower case letter (b)
– Ex. Blue eyes

Terminology
• Genotype—Genetic makeup of an organisms
– Homozygous—2 alleles that are the same (BB or bb)
– Heterozygous—2 different alleles (Bb)
• Phenotype—Observable characteristics
– Brown eyes, brown hair, tall, short
• Generation—group of organisms born around the
same time
– P Generation—Parental
– F1—offspring of parent, first filial generation
– F2—1st generation of offspring

Genetic Terminology
Term Definition
Generations
P First (parental) generation
F1 children (Latin filii) of
generation P
F2 children of generation F1
Chromosomes and Genes
chromosome macromolecules found in
cells, consisting of DNA,
protein and RNA
gene the unit of heredity of a living
organism
locus specific location of a gene,
DNA sequence, or position on
a chromosome
allele one of a number of
alternative forms of the same
gene or genetic locus
Term Definition
Dominant and recessive
dominant allele an allele that is always expressed if present
recessive allele an allele that is only expressed if the dominant
allele is not present
Genotypes and Phenotypes
genotype the genetic makeup of a cell, an organism, or
an individual
homozygous contains two of the same allele of a gene
heterozygous contains two different alleles of a gene
phenotype an organism's observable characteristics or
traits
true breeding pure-bred, is an organism that always passes
down a certain phenotypic trait to its offspring
wild type most common phenotype in nature
mutant physical or genetic difference from a mutation
in a gene

Tracking Inheritance
• Monohybrid Cross—Mating between individuals
that are BOTH heterozygous
• Dihybrid Cross—Mating between individuals that
are heterozygous for 2 traits (genes)
• Punnett Square—diagram showing genotypes in
a monohybrid cross
– Test cross—a Punnett square done to figure out the
genotype of an unknown
• Always use a homozygous recessive individual with unknown

Male parent
Female parent
Yy
Yy
Female gametes (1:1)
Y y
Y
y
YY
Yy
Yy yy
Genotypic ratio 1:2:1
(1 YY: 2 Yy: 1 yy)
Phenotypic ratio 3:1
(3 yellow: 1 green)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Male gametes (1:1)
Monohybrid Cross

If plant is homozygous dominant (YY):
y y
Y
y
Y
Male gametes
y
Y
y
y
Male gametes
y
YY
Female gametes
Yy Yellow seeds (Yy): 100% chance
Yy
Yy
Yy
yy
If plant is heterozygous (Yy):
Yy
Female gametes
Yellow seeds (Yy): 50% chance
Green seeds (yy): 50% chance
Yy yy
Yy yy

Law of Segregation
• Two alleles of each gene are packed into
separate gametes
– Alleles move apart during meiosis
– Occurs during meiosis 1—Metaphase
• Homologous chromosome move to opposite ends of
poles

Law of Segregation
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.
Gametes will be paired
randomly with gametes
from the other parent.

Example
1. P generation
2. F1 generation
3. F2 generation

Genetics Problem
The ability to curl your tongue up on the sides (T, tongue
rolling) is dominant to not being able to roll your tongue.
A woman who can roll her tongue marries a man who
cannot. Their first child has his father’s phenotype. What
are the genotypes of the mother, father, and child? What
is the probability that their second child will be a tongue
roller?

Law of Independent Assortment
• The segregation of alleles of one gene does
not influence the alleles for another gene
– Alleles are RANDOMLY packaged in gametes
• Since we have SO MANY genes, can’t keep
doing punnett square.
– Use Product Rule—The chance that 2
independent events will both occur = the product
of individual chances that each event will occur

Law of Independent Assortment and
Dihybrid Crosses
About 70% of Americans perceive a bitter taste from the
chemical phenylthiocarbamide (PTC). The ability to taste
this chemical results from a dominant allele (T) and not
being able to taste PTC is the result of having two
recessive alleles (t). Albinism is also a single locus trait
with normal pigment being dominant (A) and the lack of
pigment being recessive (a). A normally pigmented
woman who cannot taste PTC has a father who is an
albino taster. She marries a homozygous, normally
pigmented man who is a taster but who has a mother
that does not taste PTC. What percentage of the children
will be albinos? What percentage of the children will be
non-taster of PTC?

Recessive Disorders
• Recessive Disorder—
disorder is ONLY
expressed when both
alleles are recessive
– Can be “carried”
– Heterozygous individuals
can carry gene but not
express disease
Dd Hearing carrier father
Dd
Hearing
carrier
mother
D d
D DD
Hearing
Dd
Hearing
(carrier)
d Dd
Hearing
(carrier)
dd
Deaf

Dominant Disorders
• Dominant Disorder
– Condition is expressed when
an individual has at least
one dominant allele
Achondroplasia

Autosomal Disorders
Disorder Likelihood Symptoms
Recessive Disorders
Albinism approximately 1
in 17,000
Most people with albinism appear white or very pale. Because individuals with
albinism have skin that entirely lacks melanin, their skin can burn more easily from
overexposure
Galactosemia approximately 1
in 60,000
(Caucasians)
In galactosemic patients, galactitol accumulates in body tissues. This can result in an
enlarged liver, cirrhosis, renal failure, cataracts, brain damage, and ovarian failure.
Without treatment, mortality in infants with galactosemia is about 75%.
Phenylketonuria
(PKU)
approximately 1
in 15,000 (US)
Abnormally high levels of phenylalanine accumulate in the blood and are toxic to the
brain. Complications intellectual disability, brain function abnormalities, mood
disorders, irregular motor functioning, and behavioral problems.
Sickle-cell disease approximately 1
in 5,000 (US)
Sickle-cell disease may lead to various acute and chronic complications, several of
which have a high mortality rate. Sickle-cell anaemia can lead to various
complications, including: increased risk of infection, stroke, and chronic pain.
Tay–Sachs disease approximately 1
in 3,500
(Ashkenazi Jews)
Tay–Sachs disease is typically first noticed in infants around 6 months old displaying
an abnormally strong response to sudden noises or other stimulus, known as the
“startle response.” There may also be listlessness or muscle stiffness (hypertonia).
Dominant Disorders
Alzheimer’s disease unknown dementia; memory loss and mental decay; not all cases are the result of genetics
Huntington’s disease approximately
5–10 in 100,000
a neurodegenerative genetic disorder that affects muscle coordination and leads to
cognitive decline and behavioral symptoms. It typically becomes noticeable in mid-adult
life.
Hypercholesterolemia unknown the presence of high levels of cholesterol in the blood; can lead to atherosclerosis and
other heart problems; can be caused by genetics or lifestyle

Variations on
Mendel’s Laws
• Incomplete
Dominance
– Individual who is
heterozygous show
intermediated
phenotype

Variations on Mendel’s Laws
• Pleiotropy
– One gene has many
effects

• Pleiotropy
– One gene has many
effects
Sickle-cells can cause many problems
in a person’s life, including organ
damage, chronic illness, and pain.

Polygenic Inheritance—Many genes control one
phenotype

Polygenic Inheritance—Many genes control one
phenotype
Read more here:
http://www.plosbiology.org/article/info:d
oi/10.1371/journal.pbio.0000069

Sex Linked Genes
• Sex Determination
– Female – XX
– Male - XY

Sex Linked Genes
• Can be carried on either X or Y chromosome
– More common on X chromosome
• Affect more males than females
Female Male
XAXa XAY

Sex Chromosomes and Sex Linked
Genes
• Eye color in fruit flies

X-Linked Disorders
Disorder Genetics Characteristics
X-Linked Recessive Inheritance
Duchenne muscular dystrophy mutation in the dystrophin gene early life muscle degeneration and eventual death
Fragile X syndrome expansion of the CGG trinucleotide
repeat on the X chromosome
most widespread single-gene cause of autism and
inherited cause of intellectual disability, especially
among boys
Hemophilia A mutant allele; ~70% of the time it is
an X-linked recessive trait, but ~30%
of cases arise from mutations
deficiency in clotting factor VIII, which causes
increased bleeding and usually affects males
Red–green color blindness absence or mutation of the genes
for red or green color receptors
difficulty with discriminating red and green hues
Rett syndrome mutations in the gene MECP2
located on the X chromosome
typically have no verbal skills, and about 50% of
individuals affected do not walk. Scoliosis, growth
failure, and constipation are very common and can
be problematic.
X-Linked Dominant Inheritance
Hypertrichosis unknown abnormal amount of hair growth over the body
X-linked hypophosphatemia mutation in the PHEX gene
sequence (Xp.22) and subsequent
inactivity of the PHEX protein
It can cause bone deformity including short stature
and genu varum (bow leggedness).
Retinitis pigmentosa (some
forms)
unknown degenerative eye disease that causes severe vision
impairment and often blindness

10. patterns of inheritance

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