Haploinsufficiency

Haploinsufficiency occurs when a diploid organism has only a single functional copy of a gene (with the other copy inactivated by mutation) and the single functional copy does not produce enough of a gene product (typically a protein) to bring about a wild-type condition, leading to an abnormal or diseased state. It is responsible for some but not all autosomal dominant disorders.

Haplosufficiency is the opposite case: when a diploid organism only has a single functional copy of a gene (with the other copy inactivated by mutation) and the single functional copy produces enough of a gene product (typically a protein) to bring about a wild-type condition.

The wild-type allele (i.e. version) of a haplosufficient gene is dominant over the mutant allele, since a heterozygote (with one mutant and one normal allele) displays the normal wild-type phenotype (i.e. is not diseased). On the other hand, the wild-type allele of a haploinsufficient gene is recessive to the mutant allele, since a heterozygote (with one mutant and one normal allele) displays the mutant (disease) phenotype. It is also possible that the heterozygote will display a third phenotype (such as diseased but of lesser severity) and in that case, the mutant allele is incompletely dominant to the recessive wild-type allele.