THE IMPERFECTIONS OF MAN 



Nor is this the whole story. Haemolytic disease can only occur 

 if the members of an interbreeding population are dissimilar in 

 their antigenic make-up. If all human beings were Rhesus- 

 positive or Rhesus-negative, it is obvious that they could not 

 get haemolytic disease (or suffer from transfusion accidents) as 

 a result of immunization by the Rh antigens. But they are, as 

 it happens, most highly diverse with regard to the antigens 

 present in their red blood corpuscles. About one in six English- 

 men lack the most mischief-making antigen of the Rh series, 

 viz. D or Rhg; about one in ten possesses the Kell antigen. 



We may now ask, what advantage do human beings enjoy 

 which compensates, or more than compensates, for their vul- 

 nerability to haemolytic disease? It is clear that man's embryo- 

 logical advantages, if such they are — a long gestation period, 

 coupled with a form of gestation which allows beneficial as well 

 as harmful antibodies to enter the foetus — could be enjoyed 

 with impunity if the entire population were either all positive 

 or all negative in respect of the blood group substances Rh 

 and Kell. The question therefore resolves itself into asking: 

 Why, then, have they not become so? Unfortunately, the 

 answer is not known; it is merely being groped after. 



Roughly speaking, there are two kinds of reasons why a 

 population should be polymorphic, i.e. should be subdivided 

 into variant types of which even the least frequent is far too 

 frequent to have originated merely through the recurrence of 

 mutations. The first is that heterozygotes (which carry and 

 therefore propagate a gene, but do not necessarily reveal its 

 presence) should stand at some special advantage relative to 

 homozygotes. Contrary to all superficial appearances, this 

 appears to be true of the blood affection known as 'sickle-cell 

 trait", the analysis of which provides a most noteworthy 

 example of a combined operation in genetics, chemistry, 

 anthropology and clinical medicine. 



Briefly, 'sickle-celP trait is an affection in which the red cells 



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