Hemizygosity

Hemizygosity 

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 X^AX^a and those of heterogametic sex have genotype X^AY they will produce, respectively, gametes X^A, X^a, and X^AY. Random union of these gametes will produce four types of individuals: X^AX^A, X^AX^a,X^AY, and X^aY. 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.