A genetic condition where there is only one copy of a gene in an otherwise diploid cell or organism.
For example, a copy of a gene that is deleted will result in hemizygosity in a diploid cell or organism. Hemizygosity is also displayed in human males where there is only one X chromosome (as the other sex chromosome is a Y chromosome). In another example, male honeybees are hemizygous as they develop from unfertilized eggs (unlike female honeybees that develop from fertilized eggs and thus have two sets of genes).
Word origin: Greek hēmi- half + zygosity.
Related forms: hemizygous (adjective)
Compare: homozygosity, heterozygosity
See also: zygosity