Blood types are determined by the presence or absence of two antigens on the surface of red blood cells: A and B. Individuals with blood type A have only A antigens, those with blood type B have only B antigens, those with blood type AB have both A and B antigens, and those with blood type O have neither A nor B antigens.

Blood type is inherited from our parents, with each parent contributing one allele for the A and B antigens. If an individual receives an A allele from one parent and a B allele from the other, they will have blood type AB. Since they have one A allele and one B allele, they are heterozygous for blood type.