Genetics Question: Bob has blood type A+, and Barb has blood type B+. Their firs

Question

Genetics Question:

Bob has blood type A+, and Barb has blood type B+. Their first child has blood type O-. The probability that their next two children consist of a boy with blood type A+ and then a girl with blood type O- is _____.

Correct answer is: 3/1024 but I am a little confused.

My logic:

Bob's blood type A+ = IA Io  and Barb"s blood type B + = IB Io

Bob & Barb would also both have Rh+ (Dd) in order for the first child to be negative.

The second is boy (1/2)AND IA Io(1/4) AND +/- (DD, Dd, Dd) (3/4) AND

The third is girl (1/2) AND Io Io (1/4) AND -/- (dd) (1/4)

In the solution the boy is supposed to have a probability of 3/4 for  IA Io but I am confused with this part because I crossed the parents in a punnet square (BO x AO) and only 1/4 was blood type AO.

Explanation / Answer

A and B groups inherited in dominant manner

The possible homozygous and heterozygous genotypes are AA, AO, BB, BO and the possible genotypes of parents are 1-AA, 2-AO, 1-OO, 1-BB, 2-BO, 1-OO

Inorder to get O group in the offspring, one of the parent should be homozygous for OO and the other parent should be heterozygous for AO

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