352 Douhleness in Stocks 



generation leaves the question still undecided. 38 F.^ singles with 

 cream plastids were tested, 33 from the mating with d-\\g\\t purple 

 and 5 from that with c^-red ; none yielded doubles in ^3. So far as 

 it goes this fact is against the view that the repulsion is only partial, 

 but again it is doubtful whether the experiment is on a sufficient scale 

 for the result to be regarded as conclusive. 



iii. One parent is homozygous and the other heterozygous in 

 regard to plastid colour. 



Mating 5. c?-sulphur- white % x no-d-cxeaxa ^ . Not yet carried 

 toF,. 



Mating 1 1. Reciprocal cross, wo-c^-cream $ x c?-sulphur- white </•. 



46 F^ families were raised, and doubles were obtained in all but one. 

 The probability that this all-single family was not derived from a cross- 

 bred has already been discussed (see p. 310). 



As we should expect, the F^ generation all have cream plastids, for 

 jP, had cream plastids, being derived presumably from the union of 

 XYw ovules with xyw pollen. The proportion of singles and doubles 

 in F2. approximates in many cases to the ratio 3 s. : 1 d. In those 

 families in which the doubles amount to more than 1 in 4 it is doubtful 

 whether the excess observed is real, but among some at least of those 

 in which the proportion is less than 1 in 4 the deficiency is probably 

 genuine (see Table IV). An explanation of these cases has already 

 been suggested (see p. 338), the supposition being (as in the case of 

 Mating 10) that we are here dealing with the additional pair of factors 

 X'Y, X' occurring in the one parent and Y in the other, the union of 

 the two producing a higher percentage of singles than is the case where 

 X and Y alone are concerned. We shall therefore express the com- 

 position of the germ cells uniting to produce F^ more fully thus 



XYX'w%xxyYw ^. 



Mating 6. c^-sulphur-white $ x wo-ci-non-cream j/. 

 Four kinds of unions of this type were made, viz. 



d-sulphur-white % x wo-rf-hoary white {incana) ^. 

 „ $ x no-d-hoa,ry red (Brompton) j/*.. 



„ $ X wo-c?-glabrous white (/*. 



„ $ X no-c?-glabrous flesh </". 



We may suppose that in this class of unions there will be at least 

 four different types of plants in Fj, and therefore that there will be 



