Haploinsufficiency

Haploinsufficiency occurs 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 of the gene 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.

Haploinsufficiency is therefore an example of genetic dominance, as a heterozygote (with one mutant and one normal allele) displays a phenotypic effect.

Human diseases caused by haploinsufficiency include:
 * Cleidocranial dysostosis
 * Huntington's disease
 * Polydactyly
 * Cancer
 * Marfan syndrome