12 Mutations and Evolution. 



probably chemical, in a locus of one chromosome, and this change 

 probably occurred during some stage of meiosis. Farther than 

 this the matter cannot be analysed at present. The altered 

 chromosome, present in the nucleus of every cell through mitosis, 

 increases the capacity of every cell for producing anthocyanin, as is 

 shown for instance by comparing cells of the chalaza in the two forms. 

 Through an unfortunate accident, only a few offspring of the 

 original novum survived. They were 9 rubricalyx, 1 rubrinervis, 

 and 2 rosettes classed as " doubtful." The next generation gave 

 three families, with ratios 10 : 5, 14 : 6 and 33 : 11 respectively, all 

 clearly representing 3 : 1 families. This leaves no reasonable 

 doubt that the original rubricalyx mutant was heterozygous for 

 one factor for red. For by no possibility can any of the three 

 ratios 10 : 5, 14 : 6 or 33 : 11 be considered to represent a 15 : 1 

 ratio, while their total, 57 : 22, is clearly close to the 3 : 1 

 expectation. Also the ratio in the previous generation (which may 

 be taken as probably 11 : 1), from which these three families were 

 derived by selfing certain individuals, while it might by itself 

 conceivably represent a 15 : 1 family, cannot do so really. Because 

 the chances are remote that, if the parent mutant were 

 heterozygous for two factors, for red, three of its offspring 

 taken at random as mother-plants of the next generation should all 

 happen to be heterozygous for a single factor (therefore giving 3: 1 

 ratios) and none of them homozygous or heterozygous for two 

 factors. 1 The evidence taken as a whole is therefore clear that 

 rubricalyx originated in one individual which was heterozygous for 

 a single factor for red. 



Duplicate Factor Mutations. 



This fact is thus emphasized because later generations produced 

 some families with 15 : 1 ratios, hence representing two factors for 

 red, in addition to families with 3 : I. 2 Furthermore, certain 15 : 1 

 families came from members of families showing a 3 : 1 ratio.* 

 These must therefore have been due to secondary mutations. The 

 evidence, part of which has been considered elsewhere, 4 shows with 

 a high degree of probability that, as we have seen, the original 



1 If the original mutant were heterozygous for two factors for red.it 

 would give in its offspring 1 plant which bred true to red, to 8 yielding 

 rubricalyx and rubrinervis im the ratio 15R : Ir, to 6 yielding ratios 3R: Ir, to 1 

 pure rubrinervis. It gave in fact three families all of which were 3 : 1. 



1 Gates 1915H. 



* Some 5: 1 ratios were also obtained, which are probably to be explained 

 by linkage. 



Gates 1915e. 



