A woman has been trying to conceive for several years, unsuccessfully. At a fertility clinic, they discover that she has blocked fallopian tubes. Using modern technologies, some of her eggs are removed, fertilized with her husband's sperm, and implanted into her uterus. The procedure is successful, but the couple discovers that their new son is color-blind and has blood type O. The woman claims that the child can't be theirs since she has blood type A and her husband has type B. Also, neither parent is color-blind, although one grandparent (the woman's father) is also color-blind.
As a genetic counselor, you would explain to the parents that
it is possible for the baby to have type O blood, since type O is inherited through a dominant allele.