Transcript Mendelian Genetics

Non-Mendelian
Genetics
• Complete dominance
• Law of segregation
• Law of independent
assortment
• One gene  one trait
Mendelian Genetics
• Codominance
• Incomplete dominance
• Multiple alleles
• Pleiotropy
• Epistasis
• Polygenic inheritance
• Sex-related inheritance
Non-mendelian Genetics
Pleiotropy
(pleion, “more”)
• One gene, many
phenotypic effects
40% of cats with
white fur and blue
eyes are deaf
Phenylketonuria (lack
of enzyme that breaks
down Phe)
Sickle cell anemia
(misshapen RBCs at
low O2 concs)
http://3.bp.blogspot.com/bUvXE8h_wuE/TycwHFuFqrI/AAAAAAAAA4
E/fWfyDR0ETNU/s1600/Phenylketonuria+(P
KU).jpg
Two genes affect coat color
• Effects of one gene are
modified by one or
several other genes
B  black, b  brown,
E  pigment deposited, e no pigment
• Examples:
–
–
–
–
Coat color in mammals
Color of wheat grains
Fruit color in squash
Bombay phenotype in ABO blood
groups in man
Epistasis
One gene affects expression of another
Bombay phenotype
• Quantitative characters
vary in a population along a
continuum or gradation
• Due to the presence of
contributory or
dominant (ABC) and
non-contributory or
recessive alleles (abc)
• Expression can be affected
by environmental factors
Polygenic inheritance
Additive effects of two or more genes on a phenotypic character
Example:
– Skin pigmentation
controlled by at least
three separately
inherited genes
– Each dark skin allele
contributes a unit of
‘darkness’ to the
individual
– AABBCC  very dark
– aabbcc  very light
(very light)
(very dark)
Aa Bb Cc x Aa Bb Cc
(medium brown)
brown
Petal length of a plant ranges
from 4mm to 12mm to
20mm. Out of 770 plants,
only 3 of them have 4mm
petals.
1. How many genes affect petal
length?
2. How many phenotypes are
present for petal length?
3. Give one genotype for a plant
with 12 mm petals.
4. Give two possible genotypes
for plants with 6 mm petals
5. What proportion of plants have
14 mm petals?
6. What is the phenotype of
plants with 7 contributory
alleles?
7. How many contributing alleles
does a plant with 8 mm petals
have?
Polygenic Inheritance
Problem Solving
•
•
sex is an inherited phenotypic
character determined by the
presence/absence of certain
chromosomes
SRY (sex-determining region of
Y) in humans
1. Sex-linked inheritance
2. Sex-limited inheritance
3. Sex-influenced inheritance
Sex-related inheritance
expression of traits is affected by the sex of the individual
• Gene linkage – genes located on the same chromosome are inherited
together
• Sex-linkage
• Sex chromosomes contain genes for many characters unrelated to sex
• X-linked/Y-linked gene
Sex-linked inheritance
Recessive alleles
• Color-blindness
• Duchenne muscular
dystrophy
• Hemophilia
• Testicular feminization
Dominant traits
• Hypophosphatemia
Examples of X-linked traits
Transmission of genes from father to son
• Testis-determining factor (TDF/SRY gene)
• Hypertrichosis of the ears
Examples of Y-linked (Holandric) Inheritance
• involves autosomal genes that
are expressed only in either
males or females
• resulting in a part or function
of the body that is present in
one sex but not the other
• Examples:
• milk production
• cryptorchidism
• feathers in domestic fowl
Genotype
Female
Male
HH
hen-feathered
hen-feathered
Hh
hen-feathered
hen-feathered
hh
hen-feathered
rooster-feathered
Sex-limited inheritance
• dominant in one sex but recessive in the other
• autosomal
• difference in expression due to the hormonal
difference between the sexes
• in heterozygotes, the expression of the trait is affected
by sex hormones
• homozygotes are unaffected and express the trait
regardless of the hormone produced
Sex-influenced inheritance
• pattern baldness
• baldness allele is
dominant in males
but recessive in
females
• a heterozygous
male is bald, but a
heterozygous
female is not
Sex-influenced
inheritance
Genotype
Female
recessive
Male
dominant
b1b1
bald
bald
b1b2
not bald
bald
b2b2
not bald
not bald