Practice question

Haploinsufficiency describes a dosage-sensitive locus where two functional copies are required for normal phenotype. Heterozygous loss-of-function deletion, nonsense or null mutation leaves only 50% protein product. For many transcription factors, structural proteins or rate-limiting enzymes, half dosage cannot sustain wild-type regulatory threshold, so abnormal development results despite one intact allele. This mechanism explains autosomal dominant inheritance of many deletion syndromes. Effect remains distinct from dominant-negative interference, and from recessive loss where 50% protein remains sufficient for normal cellular function and phenotype.