THE SPECIAL USES OF MAN 



paternity since (apart from mutation) a child with an ^ or a B 

 must have an ^4 or a jB in one of its parents. The determination of 

 blood groups on a large scale has led to the unforeseen discovery 

 of further new blood groups. Amongst these are the Rhesus scries 

 which we have already discussed as showing the unsuspected antigen 

 reactions between mother and embryo where the two differ. 



A last consequence of the study of blood groups has been derived 

 from their frequency distributions in human populations. Com- 

 parison has gradually exposed the existence in respect of the blood 

 groups of those dines which are well known for genes in equilibrium 



TABLE 31 



THE FREQUENCY OF AFFECTION IN VARIOUS RELA- 

 TIVES OF INDIVIDUALS AFFECTED WITH TUBER- 

 CULOSIS (KALLMANN AND REISNER, 1943) 



in other variable or polymorphic animals (Fig 76). The individuals 

 with B decrease in frequency from 60 per cent in parts of India to 

 10 per cent in Ireland. This gradual transition overrides the divisions 

 ol language, skin-colour and skull-shape laid down by earlier social 

 and physical studies ; and it shows the genetic gradations underlying 

 racial differences in man which knowledge of his propensities for 

 migration and hybridization, and of his variation in all large groups, 

 would lead us to expect. To this question we shall return later. 



A third respect in which the study of man has shown the way is 

 in cancer research. The importance of cancer has led to its most 

 extensive study primarily in man, but also elsewhere, with funda- 

 mental results, some of which we have already seen in relation to 

 plasmagencs and viruses. Cytologically, moreover, it has been 

 possible to show that these particles act in the development of 



351 



