Blood type is an inherited trait that depends on the blood type combination of a child's biological parents, states Baylor Scott & White Health. The A and B blood types are dominant genes and the O blood type is recessive.
Humans have two blood type genes, according to Baylor Scott & White Health. Each parent passes one of these on to his offspring. For blood type O to occur, both parents must have at least one O gene. The O blood type only occurs when a child inherits O from both parents. When a child inherits A and O or A and A, his blood genes are AO or AA, respectively, and his blood type is A. When a child inherits B and O or B and B, his blood genes are BO or BB, respectively, and his blood type is B. When a child inherits A and B, his blood genes and blood type are AB.
Blood type is important in medicine because donor and recipient blood types usually need to match, says the University of Utah Genetic Science Learning Center. Blood types A and B have antibodies on their molecules that cause a clotting reaction when given to a recipient with an unmatched blood type. Blood type O molecules do not contain these antibodies.
Two blood types are exceptions to the blood match rule, explains the University of Utah Genetic Science Learning Center. People with blood type AB can receive any type of blood because they have both type A and B blood and do not have an immune system reaction to A or B blood. People with blood type O have no antibodies that cause reactions in others and are therefore known as universal donors.