Haploinsufficiency

Haploinsufficiency is a mechanism of action to explain a phenotype when a diploid organism has lost one copy of a gene and is left with a single functional copy of that gene. The genotypic state in which one of two copies of a gene is absent is called hemizygosity. (These terms -- hemizygosity and haploinsufficiency -- are not interchangeable; one describes the genotype, while haploinsufficiency provides a potential mechanism to explain the phenotype.) The general assumption is that the single remaining functional copy of the gene cannot provide sufficient gene product (typically a protein) to preserve the wild-type phenotype leading to an altered or even diseased state. As such, hemizygosity is typically transmitted with dominant inheritance, either autosomally or X-linked in female humans.