haploinsufficiency

thumb|Haploinsufficiency model of dominant genetic disorders. A+ is a normal allele. A− is a mutant allele with little or no function. In haplosufficiency (most genes), a single normal allele provides enough function, so A+A− individuals are healthy. In haploinsufficiency, a single normal allele does not provide enough function, so A+A− individuals have a genetic disorder.

Haploinsufficiency in genetics describes a model of dominant gene action in diploid organisms, in which a single copy of the wild-type allele at a locus in heterozygous combination with a variant allele is insufficient to produce the wild-type phenotype. Haploinsufficiency may arise from a de novo or inherited loss-of-function mutation in the variant allele, such that it yields little or no gene product (often a protein). Although the other, standard allele still produces the standard amount of product, the total product is insufficient to produce the standard phenotype. This heterozygous genotype may result in a non- or sub-standard, deleterious, and (or) disease phenotype. Haploinsufficiency is the standard explanation for dominant deleterious alleles.