PHYSICAL CHARACTERS IN MAN 129 



recorded. Probably in such cases some favourable 

 condition in the transmitting parent prevents the 

 disease appearing. 



Darwin, in his chapter on Inheritance, in the 

 Variation of Animals and Plants under Domestication, 

 refers to man}^ cases of inherited abnormalities in 

 man, but the knowledge of his time was insufficient 

 to explain them, as they can now be clearly under- 

 stood in the great majority of instances. He clearly 

 recognises this when he states, *' The laws governing 

 inheritance are quite unknown." 



Differences essentially chemical are also involved 

 in many instances. The method of serum formation 

 and precipitation has been used as a means of testing 

 genetic relationships, both in animals and plants. 

 Learmonth (1920) has made certain interesting 

 observations in this connection. Human sera have 

 been classified into four groups, according to their 

 iso-agglutination* reactions. From a study of the 

 iso-agglutinins in the blood of forty families of 

 parents and children, it is concluded that tw^o 

 Mendelian factors, A and B, are concerned. Group I. 

 contains both factors, group II. contains A, group III. 

 B, and group IV. neither. These differences account 

 for the violent reactions which frequentl}'^ occurred 

 after blood-transfusion before the introduction of 

 blood-compatibility tests, since transfusion can only 

 take place safely between members of the same group. 

 With further experience the method will provide, in 

 many cases, a test of the paternity of a child. 



Among hereditary abnormalities referred to b}' 

 Windle (1891) is polymastia or accessory breasts, 



* An iso-agglutinin is an agglutinin capable of agglutinating 

 the red blood corpuscles of other individuals of the same species 

 as that in which it is developed. It is formed in the blood of an 

 animal by injection of the blood of another animal of the same 

 species. 



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