A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you whether blood group A or O-'s traits are dominant?
There are actually three different alleles that can determine an individual's blood type—A, B, and O. The alleles A and B are codominant and O is recessive to both A and B.
In this example, the father has the phenotype of blood type A and the mother has the phenotype of blood type O. In order for their daughter to inherit type O blood, the father must have been heterozygous for type A and his genotype was AO. The mother who has type O blood has the genotype OO and is homozygous recessive.
The genetic cross between these two individuals shows a 50% chance of offspring with heterozygous type A with the genotype AO and a 50% chance of homozygous type O with the genotype OO.
We actually have enough information to establish the relationship between the genes A and O where A is dominant to O.
I have included a link with different examples of genetics crosses involving blood type.