96 



THE THEORY OF THE GENE 



crossed with other species, and produced fertile off- 

 spring. These, when inbred, or back-crossed, have fur- 

 nished the only crucial evidence concerning the allelo- 

 morphic relation of genes in different species. 



Fig. 53. 

 Cross between two species of tobacco, Nicotiana Langsdorffii and 

 N. alata. In a and c the two original types of flowers are shown, 

 and in b the hybrid type. In d and e, two of the recovered types 

 in F2 are shown. (After East.) 



East crossed two species of tobacco, Nicotiana Langs- 

 dorffii and N. alata (Fig. 53). One plant with white 

 flowers was a mutant type. Despite the wide variability 

 of many characters in the second generation, the white 

 flowers appeared in one-fourth of the individuals of this 

 generation. The mutant gene of one species behaved 

 toward a gene of the other species in the same way as it 

 behaves with its own normal partner. 



Correns crossed Mirabilis Jalapa with M. longiflora. 

 A recessive mutant of Jalapa (chlorina) was used. This 



