COMPLEX GENES 



The S gene shows us just how the parts of a gene and their 

 actions must be fitted together. The evolutionary steps in building 

 up a gene must consist in replacing old parts or in putting on new 

 ones in such a way as to give the whole gene a new, and advan- 

 tageously new, action at each step. 



Fig. 83. — The Rhesus" blood groups in man. Each allelomorph reacts with three 

 of the six sera and each serum with four allelomorphs, so indicating that the 

 allelomorphs represent the combinations of three primary elements each of which 

 can exist in two alternative forms, viz. C-c, D-d, and E-e. Of the eight com- 

 binations, the one which is expected to be rarest has been discovered in Belgium 

 since this diagram was drawn: it is still unknown elsewhere. Two others occur 

 in England with frequencies of less than i per cent of gametes and only three with 

 frequencies of over 10 per cent. The common Rhesus + ^* reaction is determined by 

 D, Rhesus — ^^ being d, and the anti-Rh. serum being anti-D or A as it is sometimes 

 called. 



The constructive series formed by the natural allelomorphs of 

 the S gene help us to understand the variations in other genes. 

 Take, for example, the Rhesus blood-group gene, an ancient gene 

 which man shares with several monkeys. This gene has been repre- 

 sented as having seven main allelomorphs. With each of these, as 

 predicted by Fisher and proved by Race, there is not one, as with 

 the ABO and MiV series, but three antigenic reactions (Fig. 83). 



One of these antigens, D, carried by four allelomorphs, is of 



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