Sex Determination and Sex Linkage

Not all genes follow the simple autosomal patterns Mendel described. Genes carried on the sex chromosomes are inherited differently in males and females, producing distinctive patterns that explain why some conditions appear far more often in one sex. Understanding these patterns also allows us to interpret family pedigrees.

The Sex Chromosomes

Humans have 22 pairs of autosomes and one pair of sex chromosomes. Females are $XX$ and males are $XY$. Because females can pass on only an X chromosome while males pass on either an X or a Y, the father's gamete determines the sex of the child. A Punnett square of $XX \times XY$ predicts a $1:1$ ratio of females to males:

	X	Y
X	XX	XY
X	XX	XY

The Y chromosome is much smaller than the X and carries the SRY gene, which triggers developmen