Probability of boy and blood type B from parental genotypes

Steps:

• Children's blood types (A and O) require parents to be heterozygous AO (type A) and BO (type B); man's father's type B indicates man is BO, wife AO.
• Offspring blood types: AB, A (AO), B (BO), O (OO), each 1/4 via Mendelian segregation.
• Probability of blood type B for third child: 1/4.
• Probability third child is boy: 1/2, independent of blood type.
• Joint probability: (1/2) × (1/4) = 0.125.

Why A is correct:

• Product of independent probabilities: P(boy) = 1/2 and P(B from BO × AO parents) = 1/4 per Punnett square.

Why the others are wrong:

• B: Equals P(B blood) alone, omitting P(boy) = 1/2 factor.
• C: Overestimates by assuming either P(boy) or P(B blood) alone, ignoring joint requirement.
• D: Assumes certainty, but inheritance is probabilistic, not deterministic.

Final answer: A