86 MENDELISM chap. 



Fj plants are all fertile with dark axils. But such 

 plants in F^ give fertiles with light axils, fertiles 

 with dark axils, and steriles with dark axils in the 

 ratio I : 2 : i. No light axilled steriles appear from 

 such a cross owing to the repulsion between the 

 factor for dark axil (Z*) and that for the fertile 

 anther (F). 



These four cases have already been found in the 

 sweet-pea, and similar phenomena have been met 

 with in primulas and in peas. To certain seemingly 

 analogous cases in animals where sex is concerned 

 we shall refer later. 



Now all of these four cases present a common 

 feature which probably has not escaped the attention 

 of the reader. In all of them ike original cross 

 was such as to introduce one of the repelling factors 

 with each of the two parents. If we denote our two 

 factors by A and B, the crosses have always been 

 of the nature AAbby. aaBB. Let us now consider 

 what happens when both of the factors, which in 

 these cases repel one another, are introduced by one 

 of the parents, and neither by the other parent. 

 And in particular we will take the case in which we 

 are concerned with purple and red flower colour, 

 and with long and round pollen, i.e. with the factors 

 B and L. When a purple long {BBLL) is crossed 

 with a red round {bbll) the F^ {BbLt) is a purple with 

 long pollen, identical in appearance with that produced 

 by crossing the long pollened red with the round 

 pollened purple. But the nature of the F^ generation 

 is in some respects very different. The ratio of 

 purples to reds and of longs to rounds is in each case 

 3:1, as before. But instead of an association 



