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Whole-genome sequencing aims to sample sequence across an entire genome, rather than a selected panel or expressed subset. The label describes intended scope, not guaranteed complete observation of every base. Repeats, uneven representation and insufficient sequencing depth can leave regions unresolved. Analysis can involve mapping to a reference or genome assembly, and variants absent from the reference can require special attention. ^[ref]

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.