1. Or Why are men more likely to be defective than women?
Sex Linkage Or
Why are men more likely to be defective than women?

2. Linkage
The observation that two genes are not transmitted independently.
Why?
Two genes physically near each other on a chromosome will not assort randomly in meiosis.

3. Recombination
The frequency of recombination (crossing over) between two genes is proportional to the distance between the genes.

4. Sex-Linked Genes Sex-linked genes
Are genes located on a sex chromosome
Were discovered during studies on fruit flies
(a)
(b)
Figure 9.28

5. How to Show Sex Linkage X = X chromosome Y = Y chromosome
XH = X chromosome with H allele linked to it
Homozygous dominant female: XH XH
Heterozygous female: XH Xh
Homozygous recessive female: Xh Xh

6. Sex-linked inheritance
   
Two sex chromosomes are very different in size
Y about ¼ the size of the X
They are not genetically equivalent
Traits associated with genes on the X chromosome
- X-linked
Traits associated with genes on Y chromosome
- Y-linked

7. X-linked dominant inheritance

8. X-linked recessive traits

9. Color Blindness 8% males have red-green color blindness
red color blindness – can’t see red
green color blindness – can’t see greens
both are X-linked recessive traits
(map to the X chromosome)
Blue color blindness
autosomal recessive
(maps to chromosome 7) -

10. Red-green color blindness
Is a malfunction of light-sensitive cells in the eyes
Figure 9.30

12. Muscular Dystrophy Phenotype - progressive weakness and muscle wasting
Phenotype
- progressive weakness and muscle wasting
Duchennes Muscular Dystrophy
o X-linked recessive
o 1 in 3500 males afflicted in US
o onset between 1 and 6 yrs
o wheel chair by age 12
o death by age 20

13. Y-linked traits - genes unique to the Y chromosome
- present only in males
- from fathers to sons
- always expressed

14. Marge and Homer have decided to give Bart and Lisa a baby brother/sister. They know that “single hair syndrome” is a recessive sex linked trait. Normal hair is shaped like cotton candy. Homer has the recessive condition and Marge is a carrier. What is the chance of having a normal cotton candy type hair baby boy? Girl? What are the chances of a “single hair” boy? Girl?

15. Problem
Diagram a cross between a normal woman whose father was red green colorblind and a colorblind man.
What is the probability that their children will be affected?
What is the probability that the boys will be affected?