Genetics & Inheritance MCAT Practice Question
A 4-year-old boy is brought to the genetics clinic for evaluation of developmental delay and hypotonia. Chromosomal microarray is normal. Molecular testing reveals a point mutation in the SNRPN gene on the maternally inherited chromosome 15. His paternally inherited chromosome 15 is intact with a normal SNRPN allele. The child's mother is an asymptomatic carrier of the same mutation. Which of the following best explains why this patient does NOT express Prader-Willi syndrome despite carrying a SNRPN mutation?
Answer choices
- AThe paternal SNRPN allele is preferentially expressed, while the maternal allele is silenced through imprintingCorrect answer
- BTwo mutated alleles are required for disease expression in autosomal recessive inheritance
- CThe mutation is too subtle to cause loss of protein function and therefore is benign
- DThe mother's asymptomatic status proves the mutation is nonpathogenic in all individuals
- EHeterozygous carriers of SNRPN mutations always have incomplete penetrance
- FThe paternal allele undergoes compensatory upregulation to express twice the normal gene product
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