136 



GENETICS 



causes the former to recede from view, these nine 

 genotypes may be combined into four phenotypes as 

 follows : 



From this analysis it may be said that the Mendelian 

 ratio for a typical dihybrid is phenotypically 9:3:3:1, 

 while that for a monohybrid, as we have already seen, 



FIG. 44. Diagram to illustrate the possible combinations arising in the 

 second filial generation (F 2 ) following a cross between yellow-smooth 

 YS and green-wrinkled GW peas. 



is 3:1. This expected ratio corresponds essentially 

 with the actual results Mendel obtained in crossing 

 smooth-yellow and wrinkled-green peas. 



