The father is heterozygous for type B blood, meaning that he has one B allele and one O allele. The mother is heterozygous for type A blood, meaning that she has one A allele and one O allele.
When the two parents have children, there are several possible combinations of alleles that their offspring can inherit. The Punnett square is a useful tool for predicting the probability of each combination:
| B | O |
--|--------|-------|
A| AB | A |
--|--------|-------|
O| B | O |
In this Punnett square, each box represents a possible combination of alleles that could be inherited by the offspring. The letters on the top and side of the square represent the alleles from each parent.
The genotypic ratio of their offspring can be calculated by counting the number of boxes that have each possible combination of alleles. In this case, the possible genotypes for the offspring are AB, A, B, and O. From the Punnett square above, we can see that the genotypic ratio of their offspring is likely to be:
1 AB : 1 A : 1 B : 1 O
This means that for every four offspring they have, one is likely to have AB blood type, one is likely to have A blood type, one is likely to have B blood type, and one is likely to have O blood type. Note that the phenotypic ratio (the ratio of observable traits) may be different depending on the dominance relationships between the alleles.