33° 



THE POPULAR SCIENCE MONTHLY 



Mendel therefore concluded that individual germ cells are always 

 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 factors, or 

 causes, for red 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 doc- 

 trine from which all other of his conclusions radiate. It explains not 

 only the segregation of dominant and recessive characters from a hybrid 

 in which both are present, but also the relative numbers of pure domi- 

 nants, 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 



p a 



Fig. 



Diagram of Mendelian Inheritance, in Which the Individual is 



Represented by the Large Circle, the Germ Cells by the Small Ones, Domi- 

 nants Being Shaded and Recessives White, a, Pure dominant x pure recessive = 

 all dominant-recessives, b, Dominant-recessive x dominant-recessive = 1 pure dominant : 

 2 dominant-recessives : 1 pure recessive, c. Dominant-recessive x pure dominant = 2 

 pure dominant : 2 dominant-recessive, (L, Dominant-recessive x pure recessive = 2 domi- 

 nant-recessive : 2 pure recessive. 



dominant and the other the recessive factor, and the chance combina- 

 tion 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, 1DD :2D(R) :1RR, as shown in the ac- 

 companying diagram (Fig. 52, A, B). 



