THE MENDELIAN PRINCIPLES OF HEREDITY 77 



In other words, it was shown that the ratio of 3 yel- 

 lows to 1 green was made up of 1 pure yellow, 2 hy- 

 brid yellows, 1 pure green. This gave a clue to the 

 principles that lay behind the observed results. 



Mendel's chief claim to fame is found in the discovery 

 of a simple principle by means of which the entire 

 series of events could be explained. He pointed out 

 that if the original parent with yellow (Pi) carried 

 something in the germ that made the seed yellow, and 

 the original parent with green seeds (Pi) carried some- 

 thing that made the seed green, the hybrid should con- 

 tain both things. If both being present one domi- 

 nates the other or gives color to the pea, all the peas in 

 the hybrid generation will be of one color - - yellow in 

 this case. Mendel assumed that in the germ-cells of 

 these hybrids the two factors that make yellow and 

 green separate, so that half of the germ-cells contain 

 yellow-producing material, and half contain green- 

 producing material. This is Mendel's principle of 

 separation or segregation. It is supposed tv o.ccur 

 both in the male germ-cells of the hybrid flower, i.e. 

 in the anthers, and also in the ovules. If self-fertili- 

 zation occurs in such a plant, the following combina- 

 tions are possible : A yellow-bearing pollen grain may 

 fertilize a "yellow" ovule or it may fertilize a " green" 

 ovule. The chances are equal. If the former occurs, 

 a pure yellow-seeded plant will result ; if the latter a 

 hybrid yellow-seeded plant. The possible combina- 

 tions for the green-producing pollen are as follows : A 

 "green" pollen grain may fertilize a "yellow" ovule, 

 and produce a hybrid, yellow-seeded plant, or it may 

 fertilize a "green" ovule, and produce a green-seeded 



