290 



AiAi {I^^I^^) 

 A1A2 (7^ ^7^2) 



Ai^3(/^'7^') 

 AiO {I^H) 



A2A2 (7^2/42) 

 A2AS (7^2/43) 

 A2O {I'^H) 



A^O {I^\) [Subgroup A3 



There is no reason to believe that more subgroups of group A 

 may not be found later. There is some evidence also that there 

 may be subgroups of group B, but they have not yet been demon- 

 strated. 



The MN Series of Blood Types in Human Beings 



In addition to the four AB blood groups and their various 

 subgroups, two other agglutinogens have also been discovered, 

 the M and N agglutinogens of Landsteiner and Levine. All 

 human beings possess either or both of these agglutinogens so 

 that, with respect to these blood types, all people may be classed 

 as types M, N, or MN. These groups bear no relation to the 

 AB groups, for all three of the MN types are distributed with 

 the same frequency in each of the four AB blood groups. The 

 MN blood groups resemble the H groups in rabbits in that hu- 

 man beings only extremely rarely carry the agglutinins for these 

 antigens, but their heredity is different. By injecting blood 

 from the M, N, or MN types into rabbits, however, the corre- 

 sponding antibodies may be produced, and sera from such im- 

 munized rabbits may be used to test human blood for its par- 

 ticular agglutinogen. 



Two varieties of N agglutinogen have been found. The ag- 

 glutinogen Ni is the common N agglutinogen, but another, desig- 

 nated as agglutinogen N2, has been found in a few very rare 

 instances and differs from Ni only in that it gives an extremely 

 weak agglutination reaction. 



The MN blood types appear to be inherited as if three mul- 

 tiple alleles were operating. Gene N^ (or A"^) produces the Ni 

 agglutinogen, gene A^^ (or ^"-) produces the very rare N2 ag- 

 glutinogen, and the M agglutinogen is the result of a third allele, 

 M (or A^). Omitting from consideration the N2 agglutinogen 

 because of its rarity, the M type is homozygous for the M gene. 



