330 Doiibleness in Stocks 



Mating 3. c^-glabrous non-cream $ x c?-glabrous cream ^. 



Only one mating of this kind was made, two cream plants being 

 employed as the pollen parents (see p. 329). 



The totals from this mating were 270 singles and 281 doubles, 

 a result which agrees with the view provisionally adopted that 

 7 + a? : 9 — a; rather than 7 : 9 probably represents the true ratio of s. : d. 

 All but 8 plants were flowered and the 543 individuals recorded 

 included singles with white plastids and doubles with cream plastids, but 

 the reciprocal combinations of whiteness with doubleness and singleness 

 with creamness did not appear. Now in matings of this type all single 

 ^1 plants will presumably be derived from the meeting of XFTT ovules 

 with xyw pollen, i.e. from unions in which all three dominant factors 

 are carried by the ovules and none by the pollen. In other words the 

 union is a union between the combination white plastid colour with 

 singleness brought in by the female parent and creamness with double- 

 ness brought in by the male. Since out of a total of 548 plants in F.^ 

 all were either singles with white plastids or doubles with cream plastids, 

 it follows that redistribution of the factors in such a way as to lead to 

 the combination of singleness with creamness or of doubleness with 

 whiteness either does not occur when segregation takes place, or it 

 must occur very rarely. The above result seems to necessitate not 

 only that the F^ pollen should, as already inferred on other grounds 

 (see p. 321), all carry doubleness, but also that it should all carry cream- 

 ness. There is in fact a strong presumption that we have in these F^ 

 plants a condition similar to that obtaining in the sulphur-white race. 

 In both cases the singles result from the union of the same combinations 

 of factors {XYW % x xyw (^). If none of the pollen of the sulphur- 

 white can carry the factors XYW although all three are present in the 

 sulphur-white zygote, it need not surprise us if the same should hold 

 good for an F^ cross-bred of the same composition. The absence in F^ 

 of singles with cream plastids would thus be explained. The absence 

 of doubles with white plastids would seem to show that the factor W 

 must stand in some different relation to X and Y in the pure white 

 race to what it does in the sulphur-white. In the present case W, 

 which is introduced into the cross in combination with XY, appears 

 only to occur in combination with XY in the gametes of F^. If any 

 gametic combination is formed in which W is dissociated from X or F, 

 such as is presumed to occur in the sulphur-white, it must evidently be 

 rare, since no indication of such a gametic combination was apparent in 

 an F^ population numbering 543. 



