WHAT MAJOR GENES DO 



that they depend on a position effect, that is on the integrated action 

 of more than one genie constituent. 



Of these types, all but hypomorphs are rare. What does this 



SPERM 



A 



25°/c 



O 



70% 



^\\\\\\- B xWXVvXWSb 



Fig. 37. — The A, B, O blood group system governing blood transfusion in man. 

 The approximate frequencies of the three allelomorphs in the gametes of the English 

 population are shown along the margins of the diagram. Six genetically different 

 zygotes will then occur with frequencies proportional to the corresponding areas 

 in the diagram. A and B are, however, each dominant to O, though not to one 

 another. Thus genotypes AA and AO are indistinguishable in the phenotypes 

 they give, as are BB and BO. There are then four groups distinguishable in trans- 

 fusion behaviour, viz. AB, B, A and O, as shown by the cross-hatching. The 

 directions in which blood can be successfully transfused are shown by the arrows. 

 Group O is the universal donor and AB the universal recipient. 



mean? It means that mutation, in the major genes, is generally a 

 loss of efficiency, a breakdown in the working of a complex 

 mechanism, a pathological change in a healthy system. Thus, by a 



153 



