MENDELISM AND BIOMETRY 227 



Although it is perhaps scarcely probable that Mendel's 

 law will ultimately prove universal in its application, 

 nevertheless the few exceptions recorded by competent 

 observers still require further examination before they 

 can be accepted as invalidating the law in any single 

 instance. 



The question naturally arises as to how far the Men- 

 delian rule of inheritance agrees with or contradicts 

 those estimations of hereditary values which have been 

 arrived at by the labours of the biometricians. 



So long ago as 1902 Mr. G. Udny Yule endeavoured 

 with some apparent success to reconcile the Mendelian 

 results with those of biometry. Progress has been 

 rapid during the subsequent years, and what we have 

 now before us is rather the question of reconciling the 

 biometrical conclusions with tne firmly established 

 facts of Mendelian inheritance. 



In 1902 Yule considered the case of a pair of simple 

 Mendelian characters, A and a, exhibited in a mixed 

 population breeding together at random, in such a way 

 that the total number of germ cells bearing A and a 

 respectively might be regarded as equal in any genera- 

 tion. In such a case it will always be an even chance 

 whether a recessive parent will produce a dominant 

 or a recessive child, because the chance of its gamete 

 (a) mating with A or a is the same. A knowledge 

 of the ancestry of the recessive parent makes no 

 difference to the result. Consequently the case of 

 the pure recessive does not fall in with any possible 



theory of ancestral heredity. 



153 



