ON SUCCESSIVE DUPLICATE MUTATIONS. 217 



these families, the appearance of 15:1 ratios in the F3 of this 

 cross is of much interest, since the F2 contained no famiHes which 

 could reasonably be construed as containing duplicate factors, 

 except the one having the incomplete ratio 47:3. Reference to 

 the pedigree numbers* shows that the first two are derived 

 from the F2 family No. 60 in which the ratio is doubtful, the 

 third is derived from selfing a plant in the F2 family No. 62, 

 and the fourth from selfing one in family No. 63. In these two 

 families the ratios were respectively 67:13 and 82:13, both of 

 which are shown (p. 210) to be very near 5 :i ratios. The appear- 

 ance of these 15:1 ratios in F2 from 5:1 families can be explained 

 if we assume that independent duplicate mutations have occurred 

 in the F2 families 60, 62, and 63. This must happen as pre- 

 viously outlined, through a plant which is homozygous for one 

 factor giving rise to a plant which is heterozygous for two ; or in 

 other words, through the rearrangement of a pair of homologous 

 chromosomes so that they belong to different pairs. 



Another point which will be explained by the present hypoth- 

 esis is the difference in the depth of color in homozygous red- 

 budded races. Thus in the F3 families 93 and 95,^ containing 

 respectively 280 and 312 plants, the latter were constantly 

 darker red than the former. The latter family was doubtless 

 homozygous for duplicate factors (RRR'R'), or at least RRR'r', 

 since the family from which it was derived yielded 15:1 ratios. 

 The former family was on the other hand probably homozygous 

 for a single factor (RR) and hence not so densely red-pigmented. 



It will thus be seen that in several instances 15:1 families 

 have been obtained from the offspring of 3:1 or 5:1 families. 

 All such cases can be explained by assuming that a duplicate 

 mutation has intervened. The original mutation by which deep 

 red buds in CEnothera first appeared is an extremely rare occur- 

 rence, having occurred but once in all cultures of CEnothera. 

 When, however, a chromosome has once undergone this change 

 it is reasonable to suppose that other chromosomes in the same 

 nucleus could without difficulty take on an analogous trans- 

 formation. The whole mechanism is, however, at hand in the 



1 See "The Mutation Factor in Evolution," p. 256. 

 2i. c, p. 255. 



