EXPERIMENTAL STUDY OF HEREDITY 557 



477. Not All Dominants Alike.— But when the seeds of 

 the F2 plants, having only smooth seeds, were sown it 

 was found that the dominants were not alike, except in 

 external appearance. The seeds, though all appeared 

 smooth, carried different inheritances. One-third of 

 them {i.e., one-fourth of all the seed produced by the F2 

 generation) bred true to smoothness, being therefore pure, 

 or homozygous, for smoothness; the other two-thirds of 

 the dominants {i.e., one-half of all the seed produced) 

 again segregated in the ratio of 3 : i — one-fourth wrinkled 

 and three-fourths smooth, showing that they were hetero- 

 zygous; that is, that they still carried inheritance from 

 both the wrinkled and smooth-seeded grandparents. 



If we designate the first parental generation by P, the 

 dominant character (whatever it may be) by D, and the 

 recessive character by R, then the facts above described 

 may be diagrammed as follows: 



D9 X Rd' P (ist Parental generation) 



D (R) Fi (ist Hybrid generation) 



4 



1 T 



3D iR F2 (2d Hybrid generation) 



iD + 2D(R) 



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



478. 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 + ^, namely, a^ + 2ah + 6^, 



