E. R. Saunders 355 



Summary of results of cross-breeding. 



If we now put together the whole body of evidence obtained from 

 unions between true-breeding and eversporting forms we may summarise 

 the results as follows : 



From those matings in which the eversporting form was used as 

 5 185 F^ families were raised ; 91 showed a mixture of singles and 

 doubles, 94 were composed of singles only. This latter total no doubt 

 appears larger than it is in reality through the fact that some families 

 are probably included in it, which, if a larger sowing had been made, 

 would have been found to contain some doubles. As however it is not 

 possible to tell exactly how many of these smaller all-single families 

 should be disregarded, the totals are given as they stand. But we may 

 take the results as fully establishing the fact that when the eversporting 

 form is used as $ in matings with a true-breeding strain, some Fo 

 families will be mixed and some all single; and that the proportion of 

 the all-single to the mixed will be the same as the proportion of single- 

 to double-carrying ovules in the % parent, viz. 1 + x single : 9 — a; 

 double where x is less than 1. 



From the reciprocal form of mating 230 F^ families were obtained 

 and doubles were recorded in 227. In two at least of these exceptions 

 the evidence in regard to the seeming absence of doubles cannot be 

 regarded as conclusive, and it may be that in the remaining case the 

 same explanation also holds good ; or, it may be that this family did not 

 arise from a cross at all, but was the result of accidental self-fertilisation 

 which in this case would not betray itself in F^. 



When one of the parents in these unions is homozygous in W 

 (plastids white) and the other in w (plastids cream) the proportion of 

 F„ singles and doubles having white and cream plastids respectively 

 indicates that in almost all the Fi gametes, whether pollen or ovules, 

 each of the allelomorphs W and w is associated chiefly with the particular 

 combination of factors for singleness and doubleness with which it was 

 combined in the ovule or pollen grain used to produce F^. Thus in 

 a mating between ?io-c^-white and d-cream, W is borne for the most 

 part by those F-^ gametes carrying ZF, w by those carrying xy. Con- 

 versely when the mating is between cZ-white and ?io-c?-cream it is W 

 which is carried almost entirely by the XY gametes, w by those of 

 xy composition. 



