210 MENDEL1SM 



rapid during the last four years, and what we have now 

 before us is rather the question of reconciling the 

 biometrical conclusions with the firmly-established 

 facts of Mendelian inheritance. Quite recently Mr. 

 Yule seems to have succeeded in performing this 

 service for science, although the comments of other 

 biometrical students upon his work have still to be 

 awaited. 



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. 



But on turning to the dominant parent, the case is 

 found to be different. For such an one may be either 

 a pure dominant homozygote giving off A -gametes 

 only, or it may be a heterozygote giving off equal 

 numbers of A- and a-gametes. Yule shows that if both 

 the parents of the A individual exhibited the character 

 A , the proportionate number of its offspring which may 

 on the average be expected to show the A character is 

 greater than would have been the case if one of its 

 parents exhibited the character a. And in a similar 



