MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A geneticist is studying a family pedigree where a rare autosomal recessive disorder appears to skip generations and affects both males and females. If an affected individual (genotype 'aa') mates with a phenotypically normal individual whose sibling has the disorder, what is the probability that their first child will be affected?
- A50%
- B33.3%
- C0%
- D25%
Show answer & explanationAnswer & explanation
Correct answer: B. 33.3%
The affected individual is 'aa'. The phenotypically normal individual has an affected sibling, meaning both their parents must be carriers (Aa). Therefore, the phenotypically normal individual has a 2/3 chance of being a carrier (Aa) and a 1/3 chance of being homozygous dominant (AA). If the normal individual is Aa (2/3 probability), then the cross is Aa x aa, producing 50% affected (aa) offspring. So, the overall probability is (2/3) * (1/2) = 1/3, or 33.3%.
Why the other options are wrong
- A. This would be the probability if the normal individual was a known carrier (Aa), but their carrier status is probabilistic.
- C. This would be true if the normal individual was definitely AA, but they have a carrier risk.
- D. This would be the probability if both parents were known carriers (Aa x Aa), but one parent is 'aa' and the other is an 'at-risk' normal.
Autosomal Recessive Inheritance
Autosomal recessive inheritance describes a pattern of inheritance where two copies of an abnormal gene (one from each parent) must be present for the disease or trait to develop.
- Affected individuals typically have unaffected parents (carriers).
- Trait often skips generations.
- Males and females are affected equally.
- Probability of affected offspring from two carrier parents is 25%.
Memory trick: Calculate 'PARENT' probabilities first, then the 'CHILD'.