What is "haploinsufficiency"

n. (context genetics English) The state of a diploid organism having only a single copy of a particular gene, the other copy being inactivated due to mutation

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. Hemizygosity is not the same as haploinsufficiency; hemizygosity describes the genotype, and haploinsufficiency is a mechanism that may have caused 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.

Dominance describes the circumstance in which both alleles in a diploid organism are present but one allele is responsible for the phenotype. That genotypic state is one of heterozygosity (with two different alleles). Co-Dominance is that situation where the effects of both alleles are apparent in the phenotype.