Hemizygous organisms contain a single form (allele) of a gene without a complimentary or corresponding allele. Because the number of alleles present in an organism is referred to as the gene dosage for that organism, hemizygous organisms have the gene for which they are hemizygous in a single dose. The hemizygous state can result from the loss of chromosomal material (chromosomal deletion) or, in the normal condition, as a result of a sex chromosome linked genes, where only one copy of a chromosome may be present. A normal human male, for example, carrying only one X chromosome will contain only one allele of each gene normally found on the X chromosome.

In diploid organisms (organisms with a complete set of homologous autosomal chromosomal pairs), such organisms are hemizygous for a particular gene if there is only one copy of a particular gene that codes for a specific character...