Hemizygous

The condition in which genes are present only once in the genotype and not in pairs. Occurs for all genes in haploid s, for all genes located in the differential segments of the sex chromosomes in diploids, and in various aneuploids and deletion mutant heterozygote s. Synonym: haplozygous.

Having only one copy of a particular gene. For example, in humans, males are hemizygous for genes found on the Y chromosome.