X-Linked Traits in Humans

Let us first examine traits determined by genes on the X-chromosome, or the X-linked traits. The human X chromosome contains many genes that are required in both the sexes, whereas the Y chromosome contaias only a few genes, principally the genes for maleness. More than two hundred traits have been found to be X-linked and only a few are known to be Y-linked. The traits controlled by genes located on the X-chromosome are also referred to as sex-linked. That is, the terms X-linked and sex-linked are used synonymously. It is more appropriate, however, to refer to these as X-linked traits, since they follow the pattern of transmission of the X-chromosome. The X-linked traits have a unique mode of inheritance because females have two doses of X-linked genes, while males have only one. Males are thus hemizygous for X-linked traits.