ON SUCCESSIVE DUPLICATE MUTATIONS. 2IJ 



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

 cross is of much interest, since the F% contained no families which 

 could reasonably be construed as containing duplicate factors, 

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

 the pedigree numbers 1 shows that the first two are derived 

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

 third is derived from selfing a plant in the Fg 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 F^ from 5:1 families can be explained 

 if we assume that independent duplicate mutations have occurred 

 in the F% 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 FS 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 RRRV, 

 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 (Enothera first appeared is an extremely rare occur- 

 rence, having occurred but once in all cultures of (Enothera. 

 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. 

 "-L. c., p. 255. 



