The ABO Blood Group

Another multiple-allele system is at the locus for the ABO blood group. This locus determines your ABO blood type and, like the MN locus, codes for antigens on red blood cells. The three common alleles for the ABO blood group locus are: IA, which codes for the A antigen; IB, which codes for the B antigen; and i, which codes for no antigen (O). We can represent the dominance relations among the ABO alle-les as follows: IA > i, P > i, IA = IB. The IA and IB alleles are both dominant over i and are codominant with each other; the AB phenotype is due to the presence of an IA allele and an IB allele, which results in the production of A and B antigens on red blood cells. An individual with genotype ii produces neither antigen and has blood type O. The six common genotypes at this locus and their phenotypes are shown in i FIGURE 5.5a.

Antibodies are produced against any foreign antigens (see Figure 5.5a). For instance, a person having blood type A produces B antibodies, because the B antigen is foreign. A person having blood type B produces A antibodies, and someone having blood type AB produces neither A nor B antibodies, because neither A nor B antigen is foreign. A person having blood type O possesses no A or B antigens; consequently that person produces both A antibodies and B antibodies. The presence of antibodies against foreign ABO antigens means

Blood-recipient reactions to donor-blood antibodies

Phenotype (blood type)


Antigen type

Antibodies made by body

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