334 Doubleness in Stocks 



combination XY. Thus, where Fi is derived from ci-non-cream 

 $ X c?-cream ^f, all or almost all the F^ pollen carries lu, whereas W 

 is carried only or chiefly by the ovules carrying XY. When, on the 

 other hand, F^ is derived from cZ-cream (/• x c^-non-cream $ it is IT 

 which is borne by all or almost all the pollen, and w only, or chiefly by 

 the ovules carrying XY. 



It remains to consider how far the scheme suggested will serve to 

 explain the results obtained when eversporting forms are bred with 

 true-breeding types. 



VI. Constitution of the zygote and segregation in the 

 pure-breeding (non-double-throwing) strains. 



The only information to be gained from the self- fertilisation of the 

 pure-breeding forms is that they breed true to singleness and to plastid 

 character. From this fact together with the evidence obtained from 

 the testing of the ovules and pollen independently by crossing, which 

 shows that the above result is not due to any complete coupling of the 

 factors concerned with either kind of germ cell, it follows that all these 

 forms are homozygous as regards X and F: those with white plastids 

 are also homozygous as regards the presence of W, those with cream 

 plastids as regards its absence (= w). A detailed consideration of the 

 results of crossing shows however that the simple formulae XYW and 

 XYw do not fully express the complex relations existing between these 

 pure-breeding strains and the eversporting forms when the two are 

 inter-crossed. It is evident from the results of these matings that 

 X and Fdo not occur under the same conditions in the pure-breeding 

 strains as in the eversporting forms ; otherwise the results as regards 

 the occurrence of doubles would presumably be the same when the 

 XY ovule of a pure-breeding individual meets the xy pollen of an 

 eversporting type, as when the XY ovule of an eversporting type is 

 either fertilised with its own pollen or with that of any other ever- 

 sporting form. But this is not the case, the percentage of doubles in 

 the one case (pure-breeding x eversporting) being always very much 

 fewer than in the other (eversporting x eversporting). 



The results of matings between typical d- and wo-d-strains in fact 

 suggest that the resulting Fi cross-breds are only able to form those 

 gametic combinations of the XxYy allelomorphs from which they 

 themselves arose; if gametes are formed carrying different combinations 

 of these factors, they must be extremely rare. Thus when an F^ 



