Traits that appear more often in one sex than the other often map to the sex chromosomes. A sex-linked trait is controlled by a gene located on the X or Y chromosome. X-linked traits are far more common because the X carries hundreds of genes while the Y carries few.

Males, having only one X, express whatever allele they inherit on that chromosome. Females need two copies of a recessive X-linked allele to show the trait. Classic examples include red-green color blindness and hemophilia. Y-linked traits pass strictly from father to son.

Pedigrees of X-linked recessive conditions show no male-to-male transmission and a higher proportion of affected males. Carrier females may show mild symptoms because of skewed X inactivation. Genetic counseling for sex-linked disorders requires careful attention to the sex of each family member and the distinctive transmission rules.

• Trait controlled by genes on sex chromosomes
• X-linked traits more common than Y-linked
• Males express single X-linked alleles
• Distinctive pedigree patterns