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 X-linked 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 “non-

bald.” 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 sex-linked 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