Download Sex linkage

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 lightsensitive
cells in the
eyes
Figure 9.30
Muscular Dystrophy
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?