DIHYBRIDISM 



203 



case wil be readily understood from the accompanying table 

 (Fig. 81) : 



R Y 



RG 



W Y 



WG 



FIG. 81. Dihybridism. 



The constitution of the hybrid (in so far as the characters in 

 question are concerned) is E (G) (W) Y. Each of its germ cells, 

 however, contains one character, and one only, from each con- 

 trasted pair. Each may, therefore, contain E Y, E G, W Y or 

 W G. These different kinds of germ cells will occur on the 

 average in equal numbers, and on self-fertilization of the hybrid 

 flowers they will unite in pairs at random. The possible 

 ways in which such union may take place are shown in the table, 

 and if allowance be made for the phenomenon of dominance, in 

 accordance with which G disappears from view whenever it meets 

 Y, and W whenever it meets E (as indicated by the brackets in 

 the table), we get the apparent proportion of 9 round yellow 

 seeds, 3 round green seeds, 3 wrinkled yellow seeds, and 1 wrinkled 

 green seed. The number of wrinkled green seeds will be small at 

 first, but they will breed true, containing as they do only the 

 extracted recessive characters, for if a dominant character were 

 present it would necessarily show itself. 



Thus we see that one way of producing new forms of plants 

 and animals is by the artificial combination of characters 

 which already exist in different varieties. These elementary or 

 unit characters can be brought together by the process of hy- 

 bridization, and new organisms produced in somewhat the same 



