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MBBS QUESTION #10155
Question 1
Two clinically unaffected parents, both known heterozygous carriers of an autosomal recessive disorder, ask for the probability that their next child will be affected. Using standard Mendelian inheritance for an Aa × Aa cross, what is this probability?
  • $\frac{1}{2}$
  • $\frac{1}{4}$✔️
  • $\frac{1}{8}$
  • $\frac{3}{4}$
Correct Answer Explanation
An Aa × Aa cross produces offspring in the genotypic ratio $1:2:1$ (AA : Aa : aa). Only the homozygous recessive (aa) genotype is affected, giving a probability of $\frac{1}{4}$ for an affected child — the basis of genetic counselling for autosomal recessive conditions.