COUPLING 201 



complicated a proportion as this in the absence of 



independent considerations. 



[In the case of other characters, the coupling between 

 members of distinct pairs was closer, and suggested 

 gametic ratios approaching 15 : i : i : 15 or 31 : i : i : 31, 

 so that Bateson and Punnett have supposed that the 

 sum of the series is always an exact power of 2 

 (8, 16, 64, etc.), and that the phenomenon is due to a 

 differential division of a germ-cell followed by more 

 divisions of one of the daughter cells than of the other. 

 If, for example, a primitive germ-cell containing all 

 four factors those for oval and round pollen, blue and 

 red flowers divides twice in such a way that the four 

 cells produced contain respectively the factors blue 

 and oval, blue and round, red and oval, red and round, 

 and then the cells bearing blue-oval and red-round 

 divide, while the others do not, the observed excess of 

 germ-cells bearing these characters will be accounted 

 for. But Professor Morgan in Drosophila does not find 

 these ratios of 7 : i, 15 : i, etc., and has offered an en- 

 tirely different explanation of the facts which will be 

 mentioned in the chapter on Cytology (Chapter IX.). 



It should be noticed that characters which are 

 coupled are always introduced into the double hetero- 

 zygote from the same parent. If, in the Sweet Pea, 

 one parent is blue -oval, the other red-round, the blue 

 flower will be coupled with the oval pollen and the red 

 flower with the round pollen in the grandchildren. 

 But if such characters are introduced from different 

 parents, the converse phenomenon of gametic repulsion 

 is seen. If a blue-flowered round-pollened plant is 



