The X chromosome is a sex chromosome in the human and many other mammalian chromosome systems. Typical human XX cells contain two copies and typical XY cells contain one. Many genes on the X chromosome participate in functions beyond sex determination, so its inheritance can influence a wide range of phenotypes.[ref]
For an illustrative recessive allele in a region without an equivalent functional copy on the Y chromosome, an XY individual has only the one X-linked copy at that location. An XX individual can carry another allele on the other X chromosome, although X-chromosome inactivation and other biological details affect expression. Thus “recessive” describes a relationship between alleles when both are present; it does not make a single inherited copy biologically invisible. The example explains an inheritance pattern without making every trait on the X chromosome follow one simple rule.
