21. Person #2 is of genotype. By looking
at the children from her & #3, we can see one child has Type O blood, so
she must have an O allele for the sun to have Type O blood. One of the sons
is Type B, and another is Type AB. They get the A from the dad (person #3),
so she must be giving them the B. This would give person #2 the genotype of
BO.
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Slichter