E. R. Saunders 351 



a small proportion of the whole number of gametes. On the supposi- 

 tion that the repulsion is only partial we might expect recombination 

 thus : 



Ovules Pollen 



?i - 1 XYw n - 1 XYw 



1 XYW 1 XYW 



1 xyiu 1 xyw 



n 



— \ xyW n — \ xyW 



This would give a result in F.^ almost precisely similar to that 

 produced by complete repulsion, except that there would be in addition 

 to the three forms given above a proportion of rather less than 1 in 

 1000 of doubles with cream plastids, if n= 16; or rather more than 

 1 in 4000 if n = 32. The difficult}' of distinguishing between these 

 two possibilities will be apparent from an examination of the figures 

 given below, where the composition of the resulting F^ generations is 

 compared in detail in the two cases. 



Totals ... 1024 1024 4096 4096 



As between complete repulsion on the one hand, or partial repulsion 

 on either a 1.5 : 1 or a 31 : 1 basis on the other, the evidence therefore 

 is not absolutely clear. If the former assumption (coupling complete) 

 should prove correct, then, as stated above, all F.2 singles with white 

 plastids should prove heterozygous in singleness and doubleness and 

 also in plastid character ; while all the singles with cream plastids 

 should be homozygous in both characters : further, the observed absence 

 of doubles with cream plastids will be absolute. If on the other hand 

 the repulsion is partial, then certain of the F.y singles with white plastids 

 will breed true both to singleness and to whiteness, others to singleness 

 though not to whiteness, others to whiteness though not to singleness, 

 while others again will be heterozygous as regards both characters : 

 similarly some of the singles with cream plastids will prove to be 

 breeding pure to singleness, others not. In this case it must be 

 supposed that with a larger count in F^ an occasional double with 

 cream plastids would appear. The available evidence from the F^ 



