CHAPTER VIII 



MENDELISM GENERAL DISCUSSION 



A QUESTION of much importance is the range of applica- 

 tion of Mendel's law. Is it, as was at first befieved 

 by some, confined in its application to those cases in 

 which very distinct differences are dealt with ? Is it 

 a universal law of inheritance ? Are any real excep- 

 tions known ? 



From time to time cases have been described which 

 have been quoted as proof that Mendelian segregation 

 does not occur. Most of the apparent exceptions to 

 Mendel's law are cases in which the first hybrid genera- 

 tion is a blend, and where the blended condition appears 

 to be constantly reproduced in the subsequent genera- 

 tions. One of the most commonly cited is that of skin 

 colour in man. The children of a European and a 

 negro, mulattoes as they are termed, are just about 

 intermediate in colour. This, of course, is of little 

 importance hi itself ; but if inheritance were according 

 to the simple Mendelian scheme we ought to expect, 

 from a marriage of two mulattoes, the ratio of 1 

 " white " : 2 mulatto : 1 negro. A family of four in 

 which this ratio occurred would be sufficiently striking 

 to attract attention to say the least. It seems quite 

 certain that segregation on these lines does not occur. 

 It seems, on the other hand, that the children of mulatto 

 marriages are generally again recognisable as about 

 half-bloods, although the shade of colour is occasionally 

 somewhat lighter or darker than that of the parents. 

 It has been observed also that in marriages of two nearly 

 white people, with some negro ancestry, there is an 

 occasional reversion to a much darker shade of colour. 

 In the main, however, skin colour seems to be a character 



