An infant with type B blood is born to parents who both have type O blood. What genetic action or phenomenon is responsible for this unusual blood type expression?
A)Decreased penetrance of a dominant trait
B)Variable expressivity of a recessive trait
C)Inactivation of both parents' X chromosomes
D)Failure of one parent's blood-type gene alleles to segregate

Question

Quizplus · Accepted Answer

Penetrance is how often a gene is expressed within a population when it is present. Some AD genes have greater penetrance than others, but it is never completely 100%. Some dominant gene alleles have "reduced" penetrance. In this case (with parentage proven by other means), one parent had to have a genotype of B/O for blood type. With reduced penetrance of the B allele, he or she did not express it in his or her phenotype. However, the gene is normal and can be transmitted to the infant who did express it in the blood type phenotype.

An Infant with Type B Blood Is Born to Parents