Also found in: Medical.
the state in which one or several genes are unpaired (have no allelic partners), such as the (“sex-linked”) genes of the sex chromosomes in individuals of heterogametic sex. For example, if individuals of homogametic sex have genotype XAXa and those of heterogametic sex have genotype XAY they will produce, respectively, gametes XA, Xa, and XAY. Random union of these gametes will produce four types of individuals: XAXA, XAXa,XAY, and XaY. Of these, the second and fourth will be carriers of a alleles, but the a allele will manifest itself only in the fourth (hemizygous) individual, since there is no dominant partner. Hemizygosity may appear also as a result of chromosomal rearrangement. A number of sex-linked hereditary diseases, such as hemophilia and color blindness (caused by recessive sex-linked genes), are manifested in humans and animals as a result of hemizygosity. The principle of hemizygosity is also used in selective plant breeding by obtaining monosomes (organisms with a single unpaired chromosome) for determination of the genetic composition of chromosomes.