= X chromosome
{c}
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>.<Talking Glossary of Genomic and Genetic Terms by NHGRI>[X Chromosome entry]{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.
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