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?