E. R. Saunders 343 



3. These results can be harmonised on the assumption that in addi- 

 tion to X and T a second pair of factors X'Y' are concerned in the 

 determination of singles and doubles, as suggested above (p. 338), and 

 that these factors have a complementary distribution in some of the 

 d- and no-rf-strains^ Thus the presence of Y' in the /io-<i-strains flesh 

 and cream, and of X' in d-sulphur-white and c?-light purple but not in 

 rf-azure (in the case of those individuals used in these experiments), 

 would mean a higher percentage of singles in F^ than 3 s. : 1 d. after 

 crossing the flesh or cream with the two d-forms, sulphur-white and 

 light purple, but not after crossing with azure. 



We get confirmatory evidence of the genuine nature of this high 

 proportion of singles from the results in F^. We should expect from 

 analogy with simpler cases that Fi would behave diflferently from F^ in 

 that not all the singles would yield a mixed offspring but that about 

 one-third would prove to breed true to singleness. Those ^2 singles 

 which yielded a mixture would presumably give the same proportions 

 as the ^1 plants. This was found to be the case in the one kind of 

 mating in which the experiment was carried to F^. In the case of the 

 mating no-d-glabrous flesh % x c?-glabrous light purple ^T, 14 ^2 singles 

 were self-fertilised to produce ^3. Disregarding one family of 8 singles 

 as indecisive we find that among the remaining 13 families 4 were 

 composed entirely of singles, and 9 included a mixture of singles and 

 doubles; in two cases a proportion of about 3 s. : 1 d. was recorded, in 

 the other 7 the proportion of singles was considerably higher. 



ii. The parents are of unlike plastid colour ; each is homozygous in 

 the allelomorph concerned ( TT or lu). 



In these cases we have to consider not only the total number of 

 singles and doubles obtained, but also the proportion of each form 

 having white and cream plastids respectively. 



^ A somewhat similar case in which the recessive form was found to occur in an 

 extremely small proportion in F*, owing to the presence of several factors in Fi, any one 

 of which alone sufficed to produce the dominant form, has already been investigated and 

 fully described by Nilsson-Ehle. This observer finds that if two wheats are crossed 

 together one having red grains and the other white, plants with white grains only occur in 

 Fn in the proportion of 1 in 64. This, he explains, is due to the existence in the red 

 wheat of three factors (i?i , R-i, R^, the presence of any one of which will suffice to render 

 the grain red. Hence only those F^ plants in which all three factors are absent will have 

 white grains, and these will only occur in the proportion of 1 in 64. (See Nilsson-Ehle, 

 Kreuzungtuntersuchungen an Hafer und Weizen, Lund, 1909.) The Stocks appear to offer 

 a parallel but more complex case, as in this instance pairs of factors instead of single 

 factors are concerned. 



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