So I was solving some biology questions, and this question left me confused. The response is supposed to be 22% according to the answers, but all the methods that I used did not lead me to it. I really hope that somebody can help me with it, the question sounds as follows:

Human ABO blood type is determined by two genes (H and I). First, the H gene codes for the antigen precursor. The dominant allele (H) leads to expression of the precursor; the recessive allele (h) does not. Second, the I gene has three allele forms, IA, IB and i, and determines blood type (A, B, O or AB).

A male with blood type A and a female with blood type B marry. Each of them is heterozygous for both the H gene and the I gene. What is the probability of having a son with blood type O? Give your answer as a percentage (%) rounded to an integer (without any decimals).


7/16 for any child to be blood group O, half of that (21.875%) for a son with blood type O.

P x M
hA hB
hO hO

HA = 0/4 blood type O
HO = 2/4 blood type O (HO-HO, HO-hO)
hA = 2/4 blood type O (hA-hB, hA-hO)
hO = 3/4 blood type O (hO-HO, hO-hB,hO-ho)
= 7/16 chance of type O

son = 1/2

Chance of blood group O = 7/16 * 1/2 = 7/32 = ~0.22

Edit: as it's not clear from the question, anyone who has genotype hh is not able to express either A or B antigens, so is always blood group O.

  • 3
    $\begingroup$ Welcome to the site, Greg. I wanted to make sure you're aware of the homework policy on the site: biology.stackexchange.com/help/homework For homework questions, we want to make sure people are providing their own reasoning and work and that answers to these questions are about explaining the approach someone might take to a problem rather than answering it for them. Otherwise there's a high risk that people will ask questions here just to get the answer and won't really learn from it. $\endgroup$
    – Bryan Krause
    May 19 at 15:58

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