E. R. Saunders 843 



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

 tion to X and Y 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 ?;o-(^-strains'. Thus the presence of Y' in the ;io-rf-strains flesh 

 and cream, and of X' in rf-sulphur-white and d-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 ^2 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 F3. We should expect from 

 analogy with simpler cases that F.^ would behave differently 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 F^ singles 

 which yielded a mixture vvoidd presumably give the same proportions 

 as the F^ 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-cZ-glabrous flesh $ x rf-glabrous light purple </, 14 F„ singles 

 were self-fertilised to produce F-^. 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 (TTor 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 Fo, owing to the presence of several factors in Fj, 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 

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

 wheat of three factors (Ri , Ji.> , R^, the presence of any one of which will suffice to render 

 the grain red. Hence only those F2 plants in which aJl tliree factors are absent will have 

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

 Kreuzungsuntersuchungen 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. 



23—2 



