94 MENDELISM chap. 



factors, which in these cases repel one another, are in- 

 troduced 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 Fi (BbLl) 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 F2 generation is in some respects very dif- 

 ferent. The ratio of purples to reds and of longs to 

 rounds is in each case 3 : i, as before. But instead of an 

 association between the red and the long pollen characters 

 the reverse is the case. The long pollen character is 

 now associated with purple and the round pollen with red. 

 The association, however, is not quite complete, and the 

 examination of a large quantity of similarly bred material 

 shows that the purple longs are about twelve times as 

 numerous as the purple rounds, while the red rounds 

 are rather more than three times as many as the red longs. 

 Now this peculiar result could be brought about if the 

 gametic series produced by the Fi plant consisted of 

 T BL+i Bl+ibL+y U out of every 16 gametes. Fer- 

 tilization between two such similar series of 16 gametes 

 would result in 256 plants, of which 177 would be purple 

 longs, IS purple rounds, 15 red longs, and 49 red rounds 

 — a proportion of the four different kinds very close to 



