324 



Douhleness in Stocks 



express the composition of this and other eversporting forms more 

 fully thus: 



Sulphur-white race Pure cream forms 



XxYyWw XxYyww 



ovules pollen ovules pollen ovules pollen 



15 XYW all xyW 15 XYW all xyw 15 XYw all xyw 



Zygote 

 Gametes 



Pure non -cream formal 

 XxYyWW 



1 XyW 1 XyW 



1 xYW 1 xYw 



15 xyW 15 xyw 



Expectation on self-fertilisation 



480 single whites 

 544 double ,, 



7*5 single whites 

 8-5 double ,, 



480 single whites 

 512 double creams 



32 double whites 



or 

 7'5 single whites 

 80 double creams 



•5 double whites 



1 Xyw 



IxYw 



15 xyw 



480 single creams 

 544 double „ 



7*5 single creams 

 8*5 double ,, 



These formulae would give the uniform result of 7'5 singles : 8'5 

 doubles for all eversporting forms, and would explain the occurrence of 

 a small percentage of double whites in addition to single whites and 

 double creams, and the absence of single creams when sulphur- whites 

 are self-fertilised. The fact that the number of these double whites is 

 sometimes below, rarely above, the estimated proportion of "5 in 16 or 

 about 3 per cent, is an important point in favour of the adoption of 16 

 as the value for n rather than 8. Were a 7 : 1 : 1 : 7 series taken as 

 representing the gametic output, the expectation in regard to double 

 whites would be just twice the number given above. 



We have now to see how far the assumptions made above in regard 

 to the relations existing between the factors for plastid colour and for 

 singleness and doubleness will satisfactorily explain the distribution of 

 forms with white and cream plastids among the singles and doubles 

 when the various eversporting forms are inter-crossed. 



V. Segregation in Fi cross-breds derived from two eversporting forms, 

 and statement of results obtained in Fo. 

 As shown above (p. 320) matings between two eversporting forms 

 were found to give a slight excess of doubles as in the case of self- 

 fertilisation of either of the parents. This is in accordance with the 



1 The term ' non-cream ' is used to indicate any form with uncoloured plastids 

 irrespective of the colour of the sap. 



