The A-B Blood Groups in Human Beings 289 



is complete^ dominant over 0{i). Anti-0 serum appears to 

 give weak reactions witli group B blood in a small percentage 

 of the cases and no reaction usually. This behavior has been 

 explained on the basis of an incomplete dominance of gene B 

 (I^) over 0{i). The reaction of anti-0 serum to blood of group 

 A2 is explained by supposing that the cases in which the reaction 

 occurs are individuals wht)se genotype is A2O (or I^^i) and that 

 the reaction is with the product of the 0{i) and not that of the 

 other gene. If this explanation is correct, the anti-0 (a2) serum 

 should not react with homozygous individuals of subgroup Ao, 

 whose genotype is A2A2 (or /-^^/^^). It is very difficult to iden- 

 tify such homozygotes with certainty unless both parents are of 

 group AoB, a very rare situation because of the infrequency of 

 the A2B subgroup. 



On the other hand, AViener and Karowe have suggested that 

 the anti-0 sera are related to the blood groups in the same 

 manner that the anti-Hr sera are tc the Rh blood types which 

 will be discussed later. According to this view, anti-0 sera react 

 with the properties determined by genes and Ao but not with 

 those determined by Ai and B. If so, anti-0 serum should give 

 strong reactions with 00, A2A2, and A2O individuals, weak re- 

 actions with people whose genotypes are A1A2, AiO, BO, and 

 AoB, but negative reactions with AiAi, BB, and AiB people. 



A third subgroup of group A has also been found. It has been 

 called A3, and gives a very weak reaction with anti-A sera. It 

 appears to be even rarer than subgroup A2. Another rare sub- 

 group, designated A4, has also been reported. It is characterized 

 by a very weak reaction with sera of group B. To account 

 for subgroups Ai, A2, and A3, and groups B and 0, five mul- 

 tiple alleles have been suggested. They are designated Ai, A2, 

 A-s, B, and (or /-^^, I^-, P^ P, and i). Gene 0(f) is recessive 

 to all the others. Gene B{I^) is dominant to but shows no 

 dominant-recessive relationship with any of the other three al- 

 leles, but Ai is dominant over Ao and both are dominant over ^3. 

 The relationship might be indicated as ^4.1 > ^2 > ^3 > < .B. 

 The various groups and subgroups produced by the various 

 combinations of genes (excluding the combination responsible 

 for subgroup A4) would be: 



