E. R. Saunders 345 



first case, the appropriate substitutions being made for the alternative 

 cases : 



Ovules Pollen 



n-1 XYW n-\ XYW 



I icyW I xyW 



1 XYw 1 ZTw 



M — 1 xyw 11 — 1 xyw 



where the distribution of XY, xy, W and w is the same among ovules and 

 pollen, hut ivhere partial coupling between the jjlastid colour factor and 

 the factors for singleness and douhleness occurs in such a way, that the 

 two most frequently occuri'ing terms in the series represent the combina- 

 tions received from the parents, the two rarer terms the recombinations of 

 these factors. 



The conception that heterozygotes containing the same components, 

 but having received these components combined in different ways, may 

 form different gametic series has recently been put forward by Bateson 

 and Punnett' in explanation of certain facts observed in the course of 

 their experiments with the Sweet Pea. In certain cases where two 

 separate pairs of allelomorphs are concerned, and where particular 

 combinations occurred in the gametic series with greater frequency 

 than others, they found that the results obtained would be explained 

 if it is assumed that a heterozygote of composition AaBh, which has 

 been built up of the combinations AB and ab, again forms chiefly the 

 gametes AB and ab, only comparatively few in the series being Ab or 

 aB in composition. Whereas in an AaBh heterozygote, which has 

 received A from one parent and B from the other, Ab and aB are the 

 more frequent, AB and ab the less frequent terms in the series. In 

 Stocks a parallel case may be found in the type of union now under 

 consideration, viz. those in which the parents are of unlike plastid 

 colour, each being homozygous in the allelomorph concerned {W or w), 

 and in which the one is an eversporting, the other a true-breeding single. 

 Since two factors X, Y (or X'Y') are required to produce singleness, we 

 are here concerned altogether with three factors, viz. X, Y and W, but 

 X and Y being linked together in the true-breeding single, the two 

 behave as a single allelomorph. When an XYxyWw heterozygote has 

 received these components in the combinations XYW and xyw (as in 



1 Froc. Roy. Soc. Series B, Vol. 84, p. 3, 1911. 



