64 HEREDITY AND EVOLUTION IN PLANTS 



DsXRcf P (ist Parental generation) 



D (R) Fi (ist Hybrid generation) 

 I 



■J T 



3D iR F2 (2d Hybrid generation) 



iD + 2D(R) 

 i I 



D 3D iR R F. (3d Hybrid generation) 



53. Significance of the MendeKan Ratio. — The ratio 

 3 : I or, as it appears on analysis, i : 2 : x , 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^ + 

 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 (5 + w) = 55 + 2sw + ww 



where w = wrinkling and s — absence of wrinkHng, 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^ 



that make for smoothness; the other half only those 



factors that make for wrinkling, giving 5 and w egg-cells, 



and 5 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 /actor, determiner, gene {— producer), 

 character-unit, and others, Th?se terms are essentially synonyms. 



