sex linkage or why are men more likely to be defective than women?

Sex Linkage

Or

Why are men more likely to be defective than women?

LinkageThe observation that two genes are not transmitted independently.

Why?

Two genes physically near each other on a chromosome will not assort randomly in meiosis.

RecombinationThe frequency of recombination (crossing over)

between two genes is proportional to the distance between the genes.

Sex-linked genesAre genes located on a sex chromosomeWere discovered during studies on fruit flies

Sex-Linked Genes

Figure 9.28(a) (b)

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 sizeY 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 Blindness8% 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

  Duchennes Muscular Dystrophy

o X-linked recessiveo 1 in 3500 males afflicted in USo onset between 1 and 6 yrso wheel chair by age 12o 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?