There are three alleles for blood type, all of which are found on the ABO gene. Because each person has two sets of chromosomes, there are six different genotypes possible for this characteristic.
The three alleles are commonly referred to as A, B and O. A and B are both dominant over O, so a child with type O blood must have inherited an O allele from each parent. The A and B alleles are codominant, so someone with type AB blood inherited an A allele from one parent and a B allele from the other parent. A child who inherits two A alleles has type A blood, while a child who inherits two B alleles has type B blood.
A person's blood type is determined by taking a blood sample and checking for certain antigens and antibodies. Someone with type A blood has A antigens and B antibodies. These antibodies recognize and destroy red blood cells containing incompatible blood. A person with type B blood has B antigens and A antibodies. There are no A or B antigens in people with type O blood, but there are A and B antibodies. Conversely, AB blood has no A or B antibodies, but it does have both types of antigens.