186 



GENERAL BOTANY 



Dominance is indicated in the figure by the large size of the 

 letter S, as compared with the small size of the symbol w, used 

 for the wrinkling character. 



The Mendelian ratio. Mendel's prediction that the characters for 

 both smooth and wrinkled seeds existed in the hybrid children 

 ($w) was prove'd to be correct by his next experiment. lie allowed 

 the plants produced by the hybrid children (Sw) to close-pollinate 



and produce soeds, 

 the grandchildren of 

 Fig. 90. By observing 

 and counting a large 

 number of these seeds 

 he found that the total 

 number of smooth 

 seeds bore the same 

 ratio to the total num- 

 ber of wrinkled seeds 

 as that represented 

 in the single pod in 

 the figure ; namely, as 

 3 to 1. This is now 

 known as the Men- 

 delian ratio for the 

 hybrids of the first 

 filial generation (7^, 

 children) resulting from a cross between parents that have 

 distinctive characters, as in peas and Mirabilis (Fig. 97). 



Furthermore, he found that plants grown from the ww seeds 

 always bore pure ww seeds, represented as great-grandchildren, 

 indicating that the original parental recessive strain had reap- 

 peared in the second hybrid generation (2^, grandchildren) in 

 its original purity. 



When, however, the smooth seeds were planted, the plants 

 thus produced were found to contain some pure, smooth-seeded 

 forms ($$), like the original dominant parent, and some hybrid 

 forms ($w, Sw). The grandchildren, therefore, when judged by 

 the seeds (Fig. 96), contained two strains ($$ and ww) which 



FIG. 97. Diagram illustrating crossing in Mirabilis 



The cross between the white-flowered and red-flowered 



Mirabilis jalapa (four-o'clock) gives an intermediate 



pink in F\. The usual Mendelian ratio is shown in 



F 2 . After Morgan 



