ON SUCCESSIVE DUPLICATE MUTATIONS. 2OQ 



the ratio 33:11 was undoubtedly monomerous and up till that 

 time a single mutation had taken place involving only one pair 

 of chromosomes. 



It was anticipated that the F 2 from rubricalyx X grandiflora 

 and its reciprocal would again yield 3:1 ratios but it was found 

 that in fact there were other ratios as well, the chief of which 

 now appear to be 2:1, 4:1, 5:1 and 15:1. In my discussion of these 

 extensive results 1 I was at first inclined to attribute them to an 

 effect of the g andiflora parent in modifying the frequency of 

 inheritance of the R character, and to conclude that since the 

 cross with grandiflora had obviously modified the red-bud char- 

 acter R by dilution in many cases, it must also have modified 

 the frequency with which R would appear. I have since grown 

 a large series of Fg families, the results of which are published 

 in detail elsewhere. 2 In the present communication a further 

 analysis of these F 2 and F 3 ratios will be made, from which it 

 appears that the unexpected ratios obtained in these generations 

 are probably not an effect of the cross with (E. grandiflora, 

 but they result in part from the subsequent occurrence of dupli- 

 cate mutations in rubricalyx. Other ratios, such as 5:1 cannot, 

 however, be fully explained in this way. 



In this connection it seems desirable to point out that in the 

 inheritance of any character there are two features to be taken 

 into consideration: (i) The nature of the character, and (2) 

 the mechanism of its distribution in the germ cells. Mendelian 

 writers frequently ignore the former, and biometrical writers 

 vitiate their case when they take no account of the latter; but 

 in a complete account of the inheritance of any character both 

 must be considered. As a matter of fact, although crossing with 

 grandiflora probably does not modify the mechanism of trans- 

 mission of R, yet it does seriously and permanently modify the 

 character itself in some cases, as I have shown in previous 

 publications. 



We may now consider the ratios R:r in the F2 and F$ of (E. 

 rubricalyx X grandiflora and the reciprocal. A further study of 



'Gates, R. R., 1914, "Breeding Experiments which Show that Hybridization 

 and Mutation are Independent Phenomena," Zeitschrift /. Abst. u. Vererb., n: 

 209-279, Figs. 25. 



2 "The Mutation Factor in Evolution," pp. 254 ff. 



