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Hemizygous

(Science: genetics) a situation within a cell where only one copy of a gene or group of genes is present instead of the usual two copies per gene in a diploid genome. The word is used as in the following example: the cell is hemizygous for gene X. Pertaining to diploid organisms that possess only one allele for a particular gene, an example is the single copy of genes in the sex chromosomes of male mammals, where the X and Y chromosomes are not homologous and therefore do not present matching genes against one another.