Hemizygosity

Hemizygosity is the genetic state in which a diploid organism has only one allele at a given locus, rather than the usual two. This occurs when one chromosome of a homologous pair is missing or structurally altered, or when a sex chromosome composition provides a single copy of genes in that region.

In humans, the best-known instance is males being hemizygous for X-linked genes. They possess a single copy of most genes located on the X chromosome; the Y chromosome carries relatively few genes and does not generally provide a second functional copy for X-linked loci. A small subset of X-linked genes in the pseudoautosomal regions has homologs on the Y chromosome, so those particular genes are present in two copies in both sexes. Turner syndrome (45,X) is an example where an individual has only one X chromosome, resulting in hemizygosity for many X-linked loci.

Functionally, hemizygosity influences how genetic variation is expressed. For X-linked recessive disorders, affected males typically exhibit the phenotype because there is no second X copy to mask a deleterious allele. In females, two X chromosomes and X-inactivation usually prevent full expression, making them often carriers or showing variable expression. For autosomal genes, hemizygosity can arise from deletions that remove one copy of a locus; such dosage loss can lead to haploinsufficiency, in which a single functional copy is not enough for normal function and disease can result.