214 R- RUGGLES GATES. 



carried dwarfing latent, was used to pollinate CE. grandiflora. 

 Since the F2 offspring of this cross gave 15:1 ratios in two 

 families, while the Fi was a i :i ratio, duplicate mutations must 

 have intervened between these two generations. The two 

 plants which were the parents of the families containing 142 R: 

 15 r and 113 R: 4 r respectively must have possessed the duplicate 

 factor in all their germ cells, so that they were heterozygous for 

 R and R'. Their composition might then be written RrR'r'. 



As pointed out earlier in this paper, such a condition might 

 have arisen (a) through the transformation of a chromosome 

 belonging to a second pair, (b) through an exchange of mates 

 on the part of two pairs of chromosomes. We may now examine 

 the comparative credibility of these two alternatives. There 

 are certain difficulties with either hypothesis, one of which is 

 that the transformation from the monomerous to the dimerous 

 condition, whether effected by chemical or mechanical means, 

 must apparently have taken place early in the ontogeny, before 

 definitive germ cells are formed. The alternative hypothesis 

 would be that all the germ cells had undergone the transformation 

 simultaneously and independently, which one cannot believe 

 possible. 



There is, however, one consideration which makes it appear 

 probable that the duplicate condition for R is not usually arrived 

 at through a transformation of a new chromosome, but rather 

 through a redistribution of the chromosomes. The 15:1 ratio 

 can only be obtained from an RrR'r' parent, in which both 

 duplicate factors are heterozygous. It would therefore be 

 necessary to assume when a 15:1 family is derived from a 3:1 

 family, that a chromosome belonging to a new pair had under- 

 gone a chemical transformation while its mate and the mate of 

 the original modified chromosome were unaffected, i. e., that 

 the condition Rrr'r' became altered directly to RrR'r'. This 

 is very unlikely. On the other hand, as I showed long ago,i 

 the chromosomes in CEnothera are very loosely paired during the 

 reduction division, and moreover irregular chromosome distri- 

 butions have been shown to occur at this time (as in the pro- 

 duction of CE. mut. lata). I also {I. c.) pointed out the probability 



1 "A Study of Reduction in CEnothera rubrinervis," Bol. Gazette, 46: 1-34, pis. 3, 

 igo8. 



