360 Doubleness in Stocks 



separate allelomorphs X, Y, x and y, but with this difference, that in 

 this case the same gametic series occurs among both male and female 

 gametes. That is to say the majority of both ovules and pollen in F^ 

 exhibit the combinations occurring in the ovule and the pollen grain 

 which united to produce F^ ; the rarer terms in the series are repre- 

 sented by the recombinations of the two sets of factors. Thus when 

 the mating is between single white from a «o-rf-strain of the form XYW 

 and double cream {xyw) we shall represent the gametic series in F^ thus : 



When on the other hand single cream and double white of the 

 form {XY^v) and {xyW) unite to produce F^ the gametic series will 



be as follows : 



Ovules Pollen 



n — 1 XYw n—\ XYw 



1 XYW 1 XYW 



1 xyw 1 xyiu 



?! — 1 xyW n — 1 xyW 



In the first case F„ is composed chiefly of singles with white and 

 doubles with cream plastids, the other two forms, doubles with white 

 and singles with cream plastids, being scarce. 



In the second case, on the other hand, doubles with cream plastids, 

 if they occur, must be extremely rare (none have as yet been recorded, 

 though possibly they would occur in a larger sowing) ; doubles with 

 white plastids and singles with cream plastids are fairly numerous, and 

 singles with white plastids abundant. In the sister F^ singles which are 

 homozygous in regard to singleness {XYXYWw) the distribution of 

 the plastid colour factor must be such that all or nearly all the gametes 

 of one sex carry W. Possibly the plastid colour factor W is here 

 associated with the gametes of one or other sex according as it is 

 brought in on the male or female side, as described above in the case 

 where both parents are eversporting in regard to doubleness (see 13 a 

 and h). 



