In humans, the alleles for type A and B blood are codominant. The allele for type o
blood is recessive. A mother has blood type AB and a father has blood type O.
Their child could have all of the following blood types EXCEPT
(Hint: Make a Punnett square on scratch paper.)
B is not the correct answer, Because no matter what allele the mother gives, it will be dominant over the father, who only has O. If the father had anything other than O to give then he would have A or B, assuming the second allele is O.
