A woman of blood group AB marries a man of group O. What groups can the children be, and why never AB or O?
The AB mother contributes either I-A or I-B; the O father (ii) contributes only i.
The AB mother contributes either I-A or I-B; the O father (ii) contributes only i. Children are therefore I-A i (group A) or I-B i (group B). They cannot be AB, which needs both I-A and I-B (the father supplies neither), nor O, which needs two i alleles (the mother supplies none). So every child is group A or group B.
Key point
AB x O gives only A or B children -- never AB or O -- because each parent contributes a restricted set of alleles.
Common mistake
assuming a child can simply share a parent's exact blood group.
Memory tip
write each parent's possible alleles and combine: AB x O can only make A or B.
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