MENDELISM 127 



r , while all the gametes of RR contain r . In the 

 making we get therefore : dxr=DR and r xr = 

 RR, altogether i DR + i RR. 



DR x DD = i DR + 1 DD in the same manner, while 



DR x DR= i DD % + 2 DR'+ 1 RR, as already expounded 

 in full. 



As we have found the yellow and green colour of the 

 seed to be a pair of allelomorphic characters, so there are 

 in the edible pea other such allelomorphic pairs, behaving 

 in exactly the same manner. These have been studied and 

 enumerated already by Mendel himself, who has given the 

 following pairs of dominant and recessive characters in 

 the edible pea : 



Dominant. Recessive. 



1. Seeds yellow .. .. .. green. 



2. Seeds round . . . . . . angular. 



3. Seed-coat grey (always com- ) white (always combined 



bined with purple flowers) j with white flowers). 



4. Ripe pods inflated . . . . contracted. 



5. Unripe pods green . . . . yellow. 



6. Position of flowers axial . . terminal. 



7. Length of stem tall . . . . dwarfish. 



III. FURTHER ELABORATIONS OF MENDEL'S LAW. 



While up to now we have given our attention to the 

 behaviour of one pair of allelomorphs only, leading to 

 what are called " Monohybrids," we now direct our 

 examination to cases of Dihybridism, where two pairs of 

 allelomorphs are concerned. Here each parent has two 

 characteristics, which are dominant or recessive. How 

 does the Mendelian law work out in these cases ? 



If we have two parents of edible peas once more, one 

 with two dominant characteristics let us say yellow and 

 round seeds while the other has two recessive characters 

 green and angular seeds and we denote the dominant 

 characters yellowness by A and roundness by B, while the 



