Long years of study has shown that man inherits the blood group. There are three alleles which determine the blood group. A man may possess any one or two alleles which are located in one of the 22 autosomes. No person can possess antigen-A or antigen-B without hav their corresponding genes in autosomes. Similarly, if a person in his blood possesses antigen-A and antigen-B then he must carry the alleles corresponding to both the antigens. Here both the alleles-A and B are dominant and neither of them is suppressed by the other. Because of this unusual method of inheritance it has become customary to use the letter T(Isohemagglutinin) as symbol of the genes and with a second letter as an exponent to indicate which variation of the alleles is represented. For example Iᴬ represents the gene which produces antigen-A and Iᴮ is gene which produces antigen-B.
These alleles are found either in homozygous condition or heterozygous condition. Iᴬ in homozygous condition (Iᴬ Iᴬ) produces antigen-A, Iᴮ in homozygous condition (Iᴮ Iᴮ) produces antigen-B. But both alleles in heterozygous condition (Iᴬ Iᴮ) produces antigen-A and antigen-B (A and B being both dominant) I° is recessive to Iᴬ and Iᴮ. Therefore Iᴬ I° produces antigen-A and Iᴮ I° produces antigen-B. But I° in homozygous condition (I°I°) produces neither antigen-A nor antigen-B.