64 HEREDITY AND EVOLUTION IN PLANTS 



D9 X Re? P (ist Parental generation) 



4 



D (R) Fi (ist Hybrid generation) 



4 4 



3D iR F 2 (2d Hybrid generation) 



iD + 2D(R) 



4 4 



D 3D iR R Fj (3d Hybrid generation) 



53. Significance of the Mendelian Ratio. The ratio 

 3 : i or, as it appears on analysis, i : 2 : i, is the ratio that 

 one might expect, or that might be predicted, on the basis 

 of chance. Students of algebra will recognize in it the 

 essence of the familiar square of a + b, namely, a- -f- 

 2ab + b-, where a and b each equal i. In the plants the 

 multiplication of inheritances (produced in fertilization) 

 was as follows : 



eggs (s + w) X sperms (s + u<) = ss + 2su' + li'ic 



where w = wrinkling and 5 = absence of wrinkling, i.e., 

 smoothness. 



54. Theory of Purity of Gametes. The above ratio 

 is what we would expect if half of the egg-cells and half 

 of the sperm-cells in a heterozygous plant (one of the FI 

 generation), carried only character-units or determiners 1 

 that make for smoothness; the other half only those 

 factors that make for wrinkling, giving s and w egg-cells, 

 and s and w sperm-cells in equal numbers. Therefore, in 

 pollination the chances would be equal that an s-egg would 



1 The substance or condition (protoplasmic constitution), whatever it is, 

 in the germ-cells that corresponds to any given character of the plant is 

 variously referred to by the terms factor, determiner, gene (= producer), 

 character-unit, and others. These terms are essentially synonyms. 



