MENDELISM 439 



of a pair of allelomorphs is not independent of a character in 

 another pair. 



We will cross a pea with a pale grey seed-coat, such as that 

 which formed the background for the purple and the maple 

 (many field peas exhibit no more than this grey colour), with 

 a pea with no colour at all in its seed-coat, as is the case in a 

 great number of the peas grown for the table. The result is a grey- 

 seeded pea on which there are purple spots, which existed in neither 

 parent. Heretofore this would have been labelled as an example 

 of reversion and the matter would have been regarded as settled, 

 or at any rate done with. But to call a thing reversion does not 

 make one any wiser about it. Mendelism has provided a reason- 

 able explanation of reversion in this case. Let us see how. The 

 result of breeding from these grey-coated purple-spotted hybrids 

 provided the clue. There were produced 9 grey with purple 

 spots, 3 grey, and 4 white. 



What does this mean ? The 9:3:3:1 proportion suggests 

 an explanation. It is supposed that here, as in the case of the 

 cross between the maple and the purple spot, we are dealing 

 with two pairs of allelomorphs, namely, (1) grey (G), and not- 

 grey (nG), and (2) purple spot (P) and not-purple spot (nP). 

 The grey pea exhibits the dominant grey character (G) of that 

 pair. The white pea possesses — and here we come to the case of 

 the interdependence of characters in separate pairs — the dominant 

 purple spotting (P) of that pair; but the purple spotting is not 

 exhibited because one of its properties is that it cannot be manifested 

 unless associated with the grey coat. Bearing this theory in mind, 

 let us write the 9:3:3:1 scheme for the two pair of allelomorphs 

 G and nG, P and nP. It will be : 



9GP, 3 GnP, 3 PnG, 1 nP nG. 



But we see that in the 3 P nG the purple spot is there, but the 

 grey coat is not. Therefore ex hypothesi it will not be manifested 

 and these three seeds will appear white, which gives the propor- 

 tion 9 grey with purple spots, 3 grey and 4 white. 



The possession by the hybrid of a character which neither 

 of its parents possesses is accounted for by supposing that that 

 character (in this case purple spotting) depends for its manifesta- 

 tion on two factors (in this case P and G), one of which exists in 

 one parent and the other in the other. This Mendelian hypothesis 

 therefore enables us to account for the reversion and for the 



