Dictionary of Human Evolution and Biology

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State of having or being characterized by one or more genes that have no allelic counterparts. Said of genes that are present in only one dose in a diploid genome. Found normally in haploids (e.g. certain insects) and in nonhomologous portions of the sex chromosomes. Found abnormally in diploid cells as the result of either aneuploidy or deletion of chromatid segments. Normally, the heterogametic sex (males in mammals) is hemizygous for genes on both the X and the Y sex chromosomes.

See sex-linked inheritance.

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