CHAPTER XII 



EVIDENCE AS TO MENDELIAN INHERITANCE IN MAN. 



Normal Characters — Diseases and Malformations. Domi- 

 nants — Sex-limited Dominants — Recessive s — Note on 

 Collecting Evidence. 



Of Mendellan inheritance of normal characteristics in 

 man there is as yet but Httle evidence. Only a single case 

 has been estabHshed with any clearness, namely that of 

 eye-colour. The deficiency of evidence is probably due to 

 the special difficulties attending the study of human heredity. 

 Human families are small compared with those of our 

 experimental animals and plants, and the period covered by 

 each generation is so long that no observer can examine 

 many. Now that the critical methods of study are under- 

 stood we may have every confidence that progress will be 

 made. 



In human inheritance there is however one somewhat 

 peculiar feature, the complexity of the transmission of the 

 various colour-characters^. In our experimental studies of 

 animals and plants we have rarely met with examples of a 

 descent so complex as that which the colour of hair and 

 complexion in the mixed populations of western Europe 

 certainly presents. If the colours that we see in our own 

 population followed in their descent rules so simple as those 

 traced in the mouse, or the sweet pea, or even as those 

 which a little study would undoubtedly detect in regard to 

 the colour of cats, the essential facts of Mendelism must 

 have long ago been part of the common stock of human 

 knowledge 



The case of eye-colour is comparatively simple. As 

 Hurst has shown by examining the children in a Leicester- 



* See also later with regard to albinism in man. 



