How many alleles are there for the four main types of human blood?

Three alleles.

Explanation:

Blood types are encoded thanks to three multiple alleles: IA, IB and i. Since we humans are diploids, these alleles can be combined into six different types: IAIA, IAi, IBIB, IBi, IAIB and ii. . Between alleles IA and IB, codominance occurs, ie both alleles express themselves. However, these alleles are dominant over allele i. We can represent this relationship as follows: IA = IB> i.

Human blood has four different phenotypes: type A, type B, type AB and type O, which form the well-known ABO system. The existence of these different blood types, discovered by the Austrian physician Karl Landsteiner (1868-1943) in the twentieth century, was central to the history of medicine as it allowed the development of transfusion techniques.