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 doesn't produce enough of a gene product (typically a protein) to bring about a wild-type condition, leading to an abnormal or diseased state. It's responsible for some but not all autosomal dominant disorders. Haploinsufficiency's therefore an example of incomplete or partial dominance, as a heterozygote (with one… (More on Haploinsufficiency)