The Chromosomal Theory of
Inheritance
Chapter 15
• Sex-linked
• Sex-limited
• Sex-influenced traits
Evidence for the chromosomal theory of
inheritance
• Walter Sutton – parallels between
chromosomes and Mendel’s factors
• Thomas Morgan – Mendel’s factors are on the
chromosomes
Sex Chromosomes and Inheritance
• Morgan – discovered sex chromosomes and
sex-linked genes; used fruit flies
• XY – male – heterogametic; Y contains a gene
“Sry” – sex determining region which triggers
testicular development
• X and Y have no homologous loci.
• Y encodes traits only found in males; contains
very few genes
Y encodes traits only found in males; contains
very few genes.
Genes on the Y chromosome- holandric genes
Hairy ears in men
• XX – females – homogametic
• X inactivation in females
• In embryonic cells inactivate one of
the X’s and it contracts to form a
dense body called a Barr body which
is inactive except in gonadal cells
undergoing meiosis
• Barr bodies are highly methylated (CH3 attached to the DNA)
Barr Bodies
Sex traits can be categorized into three
types of inheritance:
• sex-limited,
• sex-linked, and
• sex-influenced.
Sex-Limited Traits
• Sex-limited traits are generally autosomal
traits that are visible only within one sex.
• There are genes which influence how much
milk a lactating mother produces when she’s
nursing a baby. These genes are carried by
both males and females, but only females
ever express them.
Sex-Linked Traits
• Sex-linked traits would be considered traits
like sickle cell anemia and color blindness.
They are said to be linked because more
males (XY) develop these traits than females
(XX). This is because the females have a
second X gene to counteract the recessive
trait. Thus, the trait is more likely to be
visible in the male.
• Another interesting observation about Xlinked traits is that males always receive
their X chromosomes from their mothers,
so they also receive any X-linked traits
from their mother.
In pedigrees, sex-linked traits appear
more in males.
Sex-Influenced Traits
• Sex-influenced traits are autosomal traits
that are influenced by sex. What makes these
traits unusual is the way they are expressed
phenotypically. In this case, the difference is
in the ways the two genders express the
genes.
• If a male has one recessive allele, he will
show that trait, but it will take two recessive
for the female to show that same trait.
• One such gene is baldness.
• This gene has two alleles, “bald” and “nonbald.” The behaviors of the products of these
genes are highly influenced by the hormones in
the individual, particularly by the hormone
testosterone.
• All humans have testosterone, but males have
much higher levels of this hormone than
females do.
• The result is that in males, the baldness allele
behaves like a dominant allele, while in
females it behaves like a recessive allele.
• As in all cases, dominance only matters in the
heterozygote, so this means that heterozygous
males will experience hair loss and
heterozygous females will not. Even
homozygous females may experience no more
than a thinning of their hair, but many develop
bald spots or have receding hairlines.
Baldness
bb
Bb or bb
Problem: Will a bald female with a normal nonbald
husband have bald kids?
bb wife x BB husband
All kids will be Bb, therefore girls will
be normal (nonbald) but their boys
will eventually be bald.
How to do sex-linked crosses
• Sex chromosomes have to be indicated and
sex of offspring has to be given
• Two ways:
XAXA x XaY- nothing on the Y!
OR
AA x aY
Ex: Red (R)/white (r) eye color is inherited as a sexlinked trait in fruit flies. What could you expect from a
cross of a white-eyed female with a red-eyed male?
• Xr Xr x
XRY
XrXR all females red
XrY all males white
Sex-linked traits in humans
• Color-blindness
• Hemophilia
• Duchenne muscular dystrophy
Are you colorblind?
Did you know?
• Color Blindness, medically known as
Daltonism or deuteranopia is a common
malady, especially among men.
• There are an estimated 10 million colorblind
people driving cars in the US and they often
can't see the difference between a red and a
green light.
A person with hemophilia and a bruise.
Muscular Dystrophy