5.5 Sex-Linked Traits
Notes
Humans have 22 pairs of autosomes and 1 pair of sex chromosomes. Females are XX; males are XY.
- The father's sperm decides the child's sex, since it carries either X or Y. The chance of a boy is always 1/2.
- Sex-linked (X-linked) genes are on the X chromosome.
- Males have only one X, so a single recessive X-linked allele causes the trait. That is why X-linked recessive disorders like color blindness and hemophilia are more common in males.
- A female with one recessive allele ($X^HX^h$) is a carrier.
Practice
Why are X-linked recessive disorders more common in males?
Females need two recessive copies; males need only one.
A carrier mother ($X^HX^h$) and a father without hemophilia ($X^HY$) have a son. What is the chance the son has hemophilia?
A son gets Y from dad and either $X^H$ or $X^h$ from mom, each with chance 1/2.
A couple already has three daughters. What is the chance their next child is a boy?
Each child is a separate event, so the chance is always 1/2.
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