88 Heredity and Environment 



Mendel therefore concluded that individual germ cells are al- 

 ways pure with respect to any pair of contrasting characters, 

 even though those germ cells have come from hybrids in which 

 the contrasting characters are mixed. A single germ cell can 

 carry the factor for red flowers or white flowers, for green seeds 

 or yellow seeds, for tall stem or short stem, etc., but not for both 

 pairs of these contrasting characters. The hybrids formed by 

 crossing white and red "four o'clocks" carry the factors for both 

 white and red, but the individual germ cells formed by such a 

 hybrid carry the factors for white or red, but not for both ; these 

 factors segregate or separate in the formation of the germ cells 

 so that one-half of all the germ cells formed carry the factor for 

 white and the other half that for red. 



This is the most important part of Mendel's Law, the central 

 doctrine from which all other conclusions of his radiate. It ex- 

 plains not only the segregation of dominant and recessive charac- 

 ters from a hybrid in which both are present, but also the relative 

 numbers of pure dominants, pure recessives and mixed dominant- 

 recessives in each generation. For if all germ cells are pure with 

 respect to any particular character the hybrid offspring of any 

 two parents with contrasting characters will produce in equal 

 numbers two classes of germ cells, one bearing the dominant and 

 the other the recessive factor, and the chance combination of these 

 two classes of male and female gametes will yield on the average 

 one union of dominant with dominant, two unions of dominant 

 with recessive and one union of recessive with recessive, thus 

 producing the typical Mendelian ratio, iDD : 2D(R) : iRR, as 

 shown in the accompanying diagram and in Fig. 29 b. 



$ germ cells D .R 



I XX 1 

 $ germ cells D R 



Possible combinations I DD : 2D(R) : I RR. 



