What is haploinsufficiency?

Haploinsufficiency refers to a situation where one gene copy is not sufficient to maintain normal function. In this context, an individual typically has two copies of a gene, one inherited from each parent. When one of those copies is mutated or deleted, the remaining working copy does not produce an adequate amount of gene product (protein or RNA) to ensure normal physiological function. This can lead to various genetic disorders, where the manifestation of the disease occurs due to the insufficient dosage of the product resulting from the single functional gene copy.

In contrast, the other options describe different genetic scenarios. A situation where one gene copy is sufficient for function typically describes situations of complete dominance or normal functioning of alleles, rather than haploinsufficiency. The option related to three copies of a gene pertains to trisomy or other conditions involving aneuploidy, which is distinct from haploinsufficiency. Lastly, a scenario where both gene copies are defective would imply a different genetic situation, often involving recessive disorders rather than haploinsufficiency, where at least one functional allele is required for normal gene expression. Thus, the essence of haploinsufficiency lies in the inadequacy of a single functional gene copy to maintain normal function