EXPERIMENTAL STUDY OF HEREDITY 557 
477. Not All Dominants Alike. But when the seeds of 
the F 2 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 F 2 
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) 
4 
D (R) Fi (ist Hybrid generation) 
3D iR Fj (zd Hybrid generation) 
iD + 2 D(R) 
D 3D iR R F (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 + b, namely, a z + 2ab + &*, 
