348 Doubleness m Stocks 



mixed families with a proportion of 3 s. : 1 d. We may suppose (1) that 

 in ^2 families containing doubles all the four possible forms will occur 

 if in the non-double-throwing (^ parent W is not coupled with XY, 

 but that only the two parental forms will occur if W is linked with XY; 

 (2) that the small number of singles with cream plastids and doubles 

 with white plastids is due to partial coupling in the i^i gametes such 

 that W and w occur much more frequently in combination with the 

 factors with which they are associated in the parents than in other 

 combinations: (3) that in Fj cross-breds producing the all-single F2 

 families, all or most of the pollen must carry W, as has already been 

 seen to be the case when c?-non-cream is used as the </" parent instead 

 of wo-d-non-cream. 



Mating 9. no-d-non-cream $ x cZ-cream (/". 



This reciprocal cross was made with the same two strains. 



Summary of results. For details see Table IV. 



A true-breeding single glabrous white was crossed with pollen from 

 a double-throwing glabrous cream (plant H, see p. 314). Only four of the 

 Fi plants were tested ; each in accordance with expectation gave a mixture 

 of singles and doubles in F^. All four forms occurred, though all were 

 not recorded in each family. Singles with white plastids and doubles 

 with cream plastids were obtained from each of the ^1 cross-breds, the 

 former being in excess, but the other two forms were present in such 

 small numbers that larger sowings would be necessary to determine 

 whether their absence in the families in which they were not recorded 

 was real, or not. Quite possibly it is merely accidental. In two of the 

 families the total number of singles and doubles approximated to 3 s.: Id.; 

 in the other two the proportion of doubles was less, but the deficiency 

 is not so great (about 1 d. : 6 s.) as to render it beyond doubt that it is 

 to be regarded as genuine. 



In this form of mating Fi is presumably produced by the union 

 oi XYW ovules with xyw pollen. If gametogenesis follows the same 

 course as in the reciprocal mating where the union is between xyw 

 ovules and XYW pollen (see above, p. 344), then, taking the two families 

 in the present case where experiment gave the expected proportion of 

 3s. : Id., we should expect a total of 



Singles Singles Doubles Doubles 



with white with cream with white with cream 



plastids plastids plastids plastids 



117 2-5 (nearly) 2-5 (nearly) 37 



where 122 3 1 33 



