THE MENDELIAN PRINCIPLES OF HEREDITY 85 



An illustration will make this clear. There are races 

 of edible peas in which the surface is round ; other races 

 in which the surface is wrinkled. Mendel crossed a 

 pea that produces yellow round seeds with one that pro- 

 duces wrinkled green seeds. 



The result of this cross was a plant that produced 

 yellow round peas (Fig. 46). Both yellow and round 

 are therefore dominant characters. When these Fi 

 plants were self-fertilized, there were produced plants 

 some of which bore yellow round peas, some yellow 

 wrinkled peas, some green round peas and some green 

 wrinkled peas. These were in the proportion of 

 9:3:3:1. 



The explanation of the result is as follows : One of 

 the original plants produced germ-cells all of which 

 bore determiners for yellow and for round peas, YR ; the 

 other parent produced gametes all of which bore deter- 

 miners for green and for wrinkled, GW (Fig. 47). 

 Their combination may be represented : 



YR by GW = YRGW 



The germ-cells of the hybrid plant YRGW produced 

 germ-cells (eggs and pollen) that have either Y or G, 

 and R or W. Expressed graphically the pairs, the 

 so-called allelomorphs, are : 



Y- ' R_ 



G * W 



and the only possible combinations are YR, YW, GR, 

 GW. When pollen grains of these four kinds fall on 

 the stigma of the same kind of hybrid plant whose 

 ovules are also of the four kinds the following chance 

 combinations are possible : 



