ON SUCCESSIVE DUPLICATE MUTATIONS. 



Table I. 



207 



It will be seen that the frequency of families is very close to 

 expectation. The totals of the families containing a 15:1 ratio 

 are also very close to expectation, but for the 3:1 families the 

 agreement is not so good. The evidence seems sufficient, how- 

 ever, to justify the conclusion that two factors and two only 

 are here concerned. 



In the same way evidence is obtained (p. 25) to show that a 

 certain pure Hne (0406) has in one case a single factor for red 

 and in another case two factors. To use the terminology of 

 Lang, the race is monomerousin one case and dimerous in another. 

 In crosses between the 0406 race and 0234, which was also red,^ 

 ratios of 15:1 and 3:1 were obtained showing that two factors 

 were present, one of which must have been derived from each 

 parent. Hence the 0406 race must in this case have been 

 monomerous. In crosses between 0406 and a white race, 15:1 

 ratios were again obtained, showing that the 0406 race is now 

 dimerous. The genetic relationships of the strains used in these 

 two crosses is not stated, but a simple explanation is that in the 

 meantime the strain had undergone a second (invisible) mutation. 



No explanation of the origin of this condition was offered. 

 But there are at least two ways in which the dimerous condition 

 may have been derived from the monomerous: (i) Through a 

 mutation on the part of a second pair of chromosomes, (2) 

 through a re-mating of the chromosome pairs. Later we shall 

 compare the consequence of each of these methods of deriving 

 the duplicate condition. In the first case the duplicate mutation 

 is produced by a change very similar to that which produced the 

 original mutant. In the second case the secondary change is a 

 mechanical one, very different from the primary change which 

 was probably chemical in nature. 



I The results are given in Ber. dent. bot. Gesells., 29: 65-69, 1911- 



