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:

• Some cancers
• Cleidocranial dysostosis
• Marfan syndrome
• Polydactyly

References

• Griffiths, Anthony J. et al. (2005). Introduction to Genetic Analysis (8th Ed.). W.H. Freeman. ISBN 0-7167-4939-4
• Robinson PN, Arteaga-Solis E, Baldock C, Collod-Béroud G, Booms P, De Paepe A, Dietz HC, Guo G, Handford PA, Judge DP, et al. (2006). "The molecular genetics of Marfan syndrome and related disorders". Journal of Medical Genetics 43:769-787.