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

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.

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

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

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

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  

X-linked dominant inheritance

X-linked recessive traits

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) -

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

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

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

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?

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?