Question 1 · Genetics, Preclinical
An experimental null allele produces no protein. The other allele remains normal, but its output is insufficient and the heterozygote has the phenotype. No abnormal interfering protein is present. Which mechanism best fits?
- A. Compensated loss of function
- B. Increased normal gene expression
- C. Haploinsufficiency
- D. Dominant negative effect
- E. Gain of function
Show the answer and explanation
Answer: C. Haploinsufficiency
One functional copy does not provide sufficient normal product. That is haploinsufficiency and can make a heterozygous loss-of-function variant produce a dominant phenotype.
Why the other options are wrong
- A. Compensated loss of function: The remaining allele does not compensate: the heterozygote is affected.
- B. Increased normal gene expression: The assay shows inadequate normal output rather than increased normal expression.
- D. Dominant negative effect: The null allele produces no interfering product, excluding the described dominant negative mechanism.
- E. Gain of function: The experiment shows inadequate normal activity rather than a new or increased activity.
Key point: Haploinsufficiency means one functional gene copy is inadequate for normal function.
References
- Haploinsufficiency, National Cancer Institute, Dictionary of Genetics Terms
- Studying Gene Expression and Function, Molecular Biology of the Cell, 4th edition, Alberts and colleagues; NCBI Bookshelf