1. The Chromosomal Theory of Inheritance
Chapter 15

2. Sex-linked
Sex-limited
Sex-influenced traits

3. 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

4. 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

6. 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

7. 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)

8. Barr Bodies

9. Sex traits can be categorized into three types of inheritance:
sex-limited,
sex-linked, and
sex-influenced.

10. 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.

11. 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.

12. 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.

13. In pedigrees, sex-linked traits appear more in males.

14. 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.

15. 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.

16. 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.

17. Baldness bb
Bb or bb

18. 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.

19. 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

20. Xr Xr x XRY XrXR all females red XrY all males white
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

21. Sex-linked traits in humans
Color-blindness
Hemophilia
Duchenne muscular dystrophy

22. Are you colorblind?

24. 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.

25. A person with hemophilia and a bruise.

26. Muscular Dystrophy