228 MENDELISM 



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 of! ^4 -gametes 

 only, or it may be a heterozygote giving off equal 

 numbers of A- and 0-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 

 way a knowledge of the characters shown by the grand- 

 parents adds something to the certainty of the pre- 

 diction as to the proportionate numbers of offspring of 

 the two kinds which are to be expected, when the 

 average of a number of cases is taken according to the 

 usual statistical method. 



Yule therefore regarded the case of the dominant 

 character as showing conformity with the law of 

 ancestral heredity, according to his own statement of 

 that generalization, which was to the following effect : 

 The law that ' the mean character of the offspring can be 

 calculated with the more exactness, the more extensive 

 our knowledge of the corresponding characters of the 

 ancestry, may be termed the law of ancestral heredity.'* 



It may be remarked in passing that Yule's dis- 

 tinction of the problems of genetics into those of 

 intra-racial heredity and those of hybridization cannot 

 now be regarded as holding good, unless the term 

 hybridization is to be extended to many cases e.g., 

 tfiat of the inheritance of coat colour in thoroughbred 

 * ' New Phytologist,' vol. i., p. 202. 



