Haploinsufficiency

Haploinsufficiency occurs when one functional copy of a gene cannot produce enough product to maintain normal function. It can cause disease when the other copy is inactivated or deleted.

Gene dosage: The amount of gene product produced by one or more copies of a gene. Haploinsufficiency is a consequence of gene dosage falling below the level needed for normal function.
Autosomal dominant inheritance: Inheritance in which one altered copy of an autosomal gene can cause a trait or disorder. Haploinsufficient variants often produce disease despite an intact allele.
Allele: One of the alternative versions of a genetic sequence at a particular locus. Haploinsufficiency concerns the output remaining when one of a gene's two alleles is impaired.
Recessive allele: An allele whose associated phenotype generally appears when no functional dominant allele masks its effect. Recessive loss-of-function variants usually require impairment of both copies, unlike haploinsufficiency.
Loss-of-function mutation: A genetic change that reduces or eliminates the function of a gene product. A loss-of-function change in one allele can leave only one productive copy.
22q11.2 deletion syndrome: A congenital disorder caused by deletion of a chromosome 22 region that includes multiple genes. Reduced dosage of genes in the deleted region contributes to the syndrome's features.
Zygosity: The state of having identical or different alleles at a genetic locus. The distinction between one altered allele and two helps define haploinsufficient genotypes.
Gain-of-function mutation: A mutation that gives a gene product increased, new, or abnormal activity. It causes disease through excessive or altered activity rather than insufficient normal product.
Gene deletion: A mutation in which a segment of DNA containing one or more genes is removed. Deleting one gene copy can expose haploinsufficiency without altering the remaining copy.
Williams syndrome: A developmental disorder usually caused by deletion of one copy of a gene-rich region on chromosome 7. Its deletion removes one copy of ELN and other genes, illustrating dosage-sensitive effects.

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