Fig. 57. A trihybrid Mendelian cross between a black hooded rat (top left) and an all-yellow sport 

 (top right) recently captured among wild Norway rats in England. Fi, one of the first-generation pro- 

 geny, gray by reversion, like wild rats. F2, the eight classes of second-generation young, from left to 

 right, black hooded, black self, gray hooded, gray self, yellow self, yellow hooded, cream (non-agouti 

 yellow) self, cream hooded. Numerically these classes should beas3:9:9:27:9:3 :3:1. Let 

 the reader determine which of the eight classes may be expected to breed true and to what extent the 

 other varieties will not breed true without " fixation " (elimination of heterozygotes) . 



