THE UNIQUENESS OF THE INDIVIDUAL 



of deoxvgenated blood adopt a sickle shape. Pauling and his 

 colleagues showed that it is due to the presence of an abnormal 

 variant of adult (as opposed to foetal or infantile) haemoglobin. 

 Sickle-cell trait appears in about 9 per cent of American 

 negroes, and in a proportion varying from to 45 per cent in 

 communities of African negroes. Its inheritance, worked out 

 independently by Neel and Beet, is governed by a Mendelian 

 dominant gene, and sickle-cell trait (a condition which is not 

 harmful in itself, nor even appreciably disabling) is its hereto- 

 zygous or hybrid form of expression. Individuals who are 

 homozygous for the sickle gene, however, suffer from a grave 

 and sometimes fatal anaemia, and few of its sufferers live to 

 reproduce. 



In the face of powerful selection against the homozygous 

 form the frequency of the sickling gene is far greater than can 

 be accounted for by mutation. According to Allison, the reason 

 why it flourishes is that the heterozygote is endowed with a 

 specially high resistance to subtertian malaria; in areas of the 

 world where malaria is hyperendemic the gene is therefore 

 kept in being by the high selective advantage of the hetero- 

 zygote; elsewhere it is going or gone. 



No such neat and rounded story can be told of blood group 

 polymorphism; indeed, it was at one time thought (and by 

 geneticists feared) that the subdivision of human beings into 

 the blood groups A, B, AB, O was entirely capricious, in the 

 sense that an individual's blood group had no bearing on his 

 fitness to survive and reproduce. Workers at the British Post- 

 graduate Medical School now find that membership of blood 

 group A is associated with an increased susceptibility to cancer 

 of the stomach, and membership of group O with a greatly 

 increased susceptibility to peptic ulceration. Blood group poly- 

 morphism is thus certainly not a matter of indifference, though 

 its import is still obscure. But, so far as I am aware, no one has 

 yet been able to associate the subdivision into Rh blood types 



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