THEORY OF RESULTS 



205 



well, but these remarks will, it is Loped, help to 

 effect a preliminary orientation of the novice. 



With a view to explaining the theory to account 

 for results which follow a cross involving two pairs 

 of characters, I propose to select as an instance of 

 such a cross the classical one investigated by Mendel, 

 namely, the result of crossing a yellow wrinkled with 

 a green round pea shown in Plate III. 



A single-square Table is sufficient for the repre- 

 sentation of the union of gametes which occurs in 

 making the cross; though a four-square one might 

 have been used as on p. 175. Y is written for the 

 factor for yellow, G for green, R for round, and W for 

 wrinkled. The Table is filled up as follows : — 



(J gametes 

 YW 



(J gametes 

 GR 



03 



YG.RW 



or 



DQ 



g YW 



WD 



ot 



YG.RW 



This may be simply written as follows : — 

 YW X GR = YG.RW 



It will be seen that each gamete is represented 

 by two letters, and not one, as in the case of the cross 

 between two forms difiering in respect of one pair 

 of characters ; and each zygote by four, and not 

 two, as in the other case. This is a simple indication 

 of the fact that the two forms crossed differ in respect 

 of two pairs of characters instead of one. 



