Sex-Linked Inheritance

In humans, sex is determined by the twenty-third pair of chromosomes known as the sex chromosomes. If at conception you have two X-shaped (XX) chromosomes you are destined to be a female. If you have an X and a Y-shaped chromosome (XY) you are destined to be male. Since the X and the Y chromosomes differ in the information they carry, any gene found on the X chromosome is referred to as a sex-linked gene. Women will have two alleles for sex-linked genes while men will only have one.

Example:
Hemophilia is a sex-linked trait. A person with hemophilia is lacking certain proteins that are necessary for normal blood clotting. Since hemophilia is a recessive trait, use N for normal and n for hemophilia. A woman who is heterozygous for hemophilia (a carrier) marries a normal man. What are the possible genotypes of their offspring?

Image by Chernobyl Frog Dissections

Sex-linked inheritance can also be examined through a pedigree chart.

Example:
Examine the following pedigree chart of colorblindness. In humans, color blindness is caused by a recessive sex-linked allele. On the diagram, label the genotypes of the individuals.

Image by Chernobyl Frog Dissections