Non-Mendelian
Genetics
• Complete dominance
• Law of segregation
• Law of independent assortment
• One gene  one trait
Mendelian Genetics
• Codominance
• Incomplete dominance
• Multiple alleles
• Sex-related inheritance
• Polygenic inheritance
• Pleiotropy
• Epistasis
Non-mendelian Genetics
Non-mendelian inheritance (1st Law)
Alleles with different degrees of dominance and recessiveness
Codominance
- two dominant
alleles affect
phenotype in
separate ways
- both alleles manifest
- e.g. roan coloring in
horses – both red and
white
Incomplete
dominance
- phenotype of
offspring is between
phenotypes of two
parents
- e.g. red and white
parents give rise to
pink offspring
Multiple alleles
- genes with more
than two alleles that
control the
phenotype
- e.g. ABO blood type
system has 3 alleles-IA,IB,i. IA,IB are
codominant, i is
recessive to both
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www.emeraldgoddessgardens.com
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•
•
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
Y chromosome
• 50-60 genes
• e.g. SRY, USP9Y
X chromosome
• 800-900 genes
• wide range of
functions not
related to sex
So much more than sex
chromosomes
http://news.sciencemag.org/biology/2014/04/y-chromosome-more-sex-switch
Recessive alleles
• Color-blindness
• Duchenne muscular
dystrophy
• Hemophilia
• Testicular feminization
Dominant traits
• Hypophosphatemia
Examples of X-linked traits
If a mother was a carrier
for the recessive sexlinked gene for
defective color vision
and the father was
normal, would their sons
or daughters have
defective color vision?
Checkpoint 1
If a father and his son
both have defective
color vision, is it likely
that the son inherited
the trait from his father?
Transmission of genes from father to son
• Testis-determining factor (TDF/SRY gene)
• Hypertrichosis of the ears
Examples of Y-linked (Holandric) Inheritance
Hypertrichosis of the ear is a Y-linked trait, a
woman is married to an affected man. Is it possible
for her to have:
a) An affected son?
b) An affected daughter?
c) An affected granddaughter by her daughter?
d) An affected grandson by her daughter?
Checkpoint 2
Using
pedigrees to
detect
inheritance
patterns
1.
2.
3.
4.
5.
Autosomal dominant
Autosomal recessive
X-linked dominant
X-linked recessive
Y-linked
Checkpoint 3
Y-linked?
Dominant or recessive?
Autosomal or X-linked?
What is the genotype of individual I-1?
 Affected males have
sons who are also
affected
 Females are not
affected
Checkpoint 4
Y-linked?
Dominant or recessive?
Autosomal or X-linked?
What is the genotype of individual I-1?




Appears every generation
Males and females equally
affected
Affected offspring has at
least one affected parent
Affected parents may have
unaffected offspring
Checkpoint 5
Y-linked?
Dominant or recessive?
Autosomal or X-linked?
What is the genotype of
individual I-1?




Can skip a generation
Males and females equally affected
Affected parents only have
affected offspring
Affected offspring may have
unaffected parents
Checkpoint 6
Y-linked?
Dominant or recessive?
Autosomal or X-linked?
What is the genotype of individual
I-1?




Present every generation
More affected females than males
Affected mother  all offspring
are affected
Affected father  all daughters
are affected
Checkpoint 7
Y-linked?
Dominant or recessive?
Autosomal or X-linked?
What is the genotype of individual
I-1?





Can skip a generation
Unaffected parents may have affected
offspring
More affected males than females
Affected mother  ½ of sons are affected
Affected father  all daughters are carriers
of the trait
• 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
Rooster feathering in chickens is a trait limited in
expression only in males and determined by the
autosomal recessive genotype hh. The dominant
allele (H) produces hen-feathered males. All
females are hen-feathered regardless of genotype.
A rooster-feathered male mated to a female
produces 12 chicks: 6 hen-feathered females, 3
rooster-feathered males and 3 hen-feathered
males. What is the genotype of the female parent?
Checkpoint 8
• 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
Pattern baldness is a sex-influenced trait that is
dominant in men and recessive in women. A
heterozygous bald man marries a bald woman.
What are the phenotypic expectations for their
children and their corresponding probabilities?
Checkpoint 9