CHAP, ix REPULSION AND COUPLING 89 



One of the best illustrations of repulsion between 

 factors occurs in the sweet pea. We have already seen 

 that the loss of the blue or purple factor (B) from the 

 wild bicolor results in the formation of the red bicolor 

 known as Painted Lady (PL IV., 7). Further, we have 

 seen that the hooded standard is recessive to the ordinary 

 erect standard. The omission of the factor for the erect 

 standard (E) from the purple bicolor (PL II., 5) results 

 in a hooded purple known as Duke of Westminster 

 (PL II., 7). And here it should be mentioned that in the 

 corresponding hooded forms the difference in colour be- 

 tween the wings and standard is not nearly so marked 

 as in the forms with the erect standard, but the difference 

 in structure appears to affect the colour, which becomes 

 nearly uniform. This may be readily seen by comparing 

 the picture of the purple bicolor on Plate II. with that 

 of the Duke of Westminster flower. 



Now when a Duke of Westminster is mated with a 

 Painted Lady the factor for erect standard (E) is brought 

 in by the red, and that for blue (B) by the Duke, and the 

 offspring are consequently all purple bicolors. Purples 

 so formed are all heterozygous for these two factors, 

 and were the case a simple one, such as those which have 

 already been discussed, we should expect the F 2 genera- 

 tion to consist of the four forms : erect purple, hooded 

 purple, erect red, and hooded red in the ratio 9:3: 3:1. 

 Such, however, is not the case. The F 2 generation 



