hemizygous
(HEH-mee-ZY-gus)
A term that describes a person who has only one copy of a gene rather than the usual two copies. Hemizygosity can occur under normal conditions. For example, because most males have one X chromosome and one Y chromosome, they are hemizygous for genes on those chromosomes. Hemizygosity can also occur under abnormal conditions, such as when two copies of a gene should be present but one is missing. For example, Turner syndrome occurs when a female inherits only one copy of the X chromosome instead of the usual two copies.