Answer explanation from previous page. To make things easier, I have edited the pedigree so that only the people we need are shown AND I have indicated the genotype of the people or the options where we can’t be sure.
Following it on the paternal side (left). We know from a A x O mating that the grandfather was AO+-. The grandmother was AB+-. Therefore the father of the blue girl is EITHER AO– (A from his mother, O from his father) or AA– (A from mother, A from father). There is a 50/50 chance for each of these two genotypes.
On the maternal side, things are a little simpler. The mother has parents who are O and B, as O type blood is recessive the grandmother is OO and therefore the mother of the blue child is definitely BO.
Then you work out the probabilities. As usual you can do this in your head or with a Punnet square.
There are two potential genotypes of the father, therefore two situations.
Situation 1. If the father is AO, then there is a 1/2 chance the blue child will get the A allele from the father and a 1/2 chance they will get the B allele from mother. [1/4 chance of AB in this situation]
Situation 2. If the father is AA, then the child will definitely get an A from him, and a 1/2 chance they will get a B from their mother. [1/2 chance of AB in this situation]
There is a 50% chance of situation 1 and 50% that it is situation 2. Therefore the whole calculation becomes (1/4 x 1/2) + (1/2 x 1/2) which rationalises to (1/8) + (2/8) = 3/8. Or if you prefer percentages (25% x 0.5) + (50% x 0.5) = 12.5% + 25% = 37.5%.
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