Sex-Linked Genes Notes
Genes located on the sex chromosomes (X & Y) are said to be sex-linked genes. There are
many genes located on the X chromosome, however very few genes are found on the Y
chromosome. Many sex-linked genetic disorders have been found on the X chromosome.
Sex-linked genes are represented using XX for females and XY for males. For example,
the gene for hemophilia is an x-linked recessive disorder. The possible allele
combinations are represented as follows:
Homozygous Dominant Female without the disorder XHXH
Heterozygous Female without the disorder XHXh
Female with Hemophilia (homozygous recessive) XhXh
Male without the disorder XHY
Male with Hemophilia XhY
If you were to cross a Heterozygous female with a male with Hemophilia, the cross would
look as follows.
MALE (XhY)
Xh
Y
XH
XHXh
XHY
Xh
XhXh
X hY
Female (XHXh)
The phenotypes and genotypes of the possible offspring are:
1(25%) Heterozygous normal females (XHXh)
1 (25%) Homozygous recessive hemophiliac females (XhXh)
1 (25%) Normal male (XHY)
1 (25%) Hemophiliac male (XhY)
Since Males have only one X chromosome, they will express all x-linked genes they inherit
even if they are recessive genes. Females must possess two x-linked recessive alleles in
order to express a trait, since they have two X chromosomes.
Colorblindness is an X-linked recessive disorder. Colorblind females are rare in the
population, because they must possess two of the recessive alleles (XcXc). Males only
require ONE of the recessive alleles to express the trait (XcY), therefore colorblind males
are more prevalent in a population. In general, all X-linked recessive disorders are
more common in males than females.
Example:
Remember that colorblindness is an X-linked recessive disorder. Determine the possible
offspring of a normal vision male (XCY) and a normal vision female that is a carrier for
colorblindness (XCXc).
XC
Y
XC
Xc
What are the possible genotypes and phenotypes of the offspring?
Genotypes:
Phenotypes:
ANSWERS: Please attempt before looking at the answers. If you do not understand, please
call me over to discuss.
Genotypes: 1 XCXC; 1 XCXc; 1 XCY; 1 XcY
Phenotypes: 1 normal female; 1 carrier female; 1 normal male; 1 colorblind male