Which mechanism of autosomal dominant disorders is defined as having only a single functional copy of a gene?

• A. Gain-of-function mutations
• B. Dominant negative mutations
• C. Haploinsufficiency
• D. Functional redundancy

Answer: (C)

Haploinsufficiency is the correct mechanism for autosomal dominant disorders where a single functional copy of a gene is not sufficient to maintain normal function, leading to a phenotype. This occurs when one allele of a gene is mutated or deleted, and the remaining normal allele does not produce enough gene product (typically a protein) to meet the organism’s needs. As a result, the imbalance in gene dosage can lead to various developmental and physiological issues associated with autosomal dominant conditions.

In contrast, gain-of-function mutations result in a gene product that has an altered or enhanced function, which can contribute to disease in different ways. Dominant negative mutations refer to a scenario where the mutated protein interferes with the normal protein’s function, often by forming nonfunctional complexes. Functional redundancy describes a situation where multiple genes or pathways can achieve the same outcome, which does not directly relate to an insufficiency of one functional copy.