Sex-linked genes are genes that are physically located on either the X or the Y chromosome. Because there are very few genes located on the Y chromosome, and none that are considered to be Mendelian disease genes, the overwhelming majority of sex-linked genes are located on the X chromosome. Therefore, sex-linked traits and X-linked traits are essentially synonymous.

The distinctive nature of sex-linked traits compared to traits located on any of the 22 autosomes is related to the differences in gene copy number for sex-linked genes between males and females. Females possess two X-chromosomes and no copy of the Y chromosome. For X-linked genes, females have two copies of each (dizygosity) just as is the case for the autosomal genes and they lack genes encoded on the Y-chromosome altogether. Males, however, possess one X chromosome and one Y chromosome and they therefore have but a single copy...