(1) Characterized by having one or more genes without allelic counterparts.
(2) Pertaining to a diploid cell with only one copy of a gene instead of the usual two copies.
For example, the male is hemizygous for most X chromosome genes.
Word origin: Greek hēmi- half.
Related forms: hemizygosity (noun), hemizygote (noun).
Compare: homozygous, heterozygous