LETHAL GENES IN YELLOW MICE 399 



gene. They do not, however, breed wholly true to type, 

 for their descendants show various degrees of " hooding." 

 By selection for several generations it is possible to breed 

 a nearly white or nearly full-colored race. This result is 

 now generally believed to be due to the selection of modi- 

 fying factors which extend the size of the hood. 



Fig. 116. — Results of a Cross between two varieties of guinea-pig 

 differing in the two unit-characters, color and length of fur. Fig. a, 

 a colored and short-haired guinea-pig. Fig. b, an albino and long- 

 haired guinea-pig. The Fi young were colored and short-haired like 

 the parent shown in Fig. a. Fig. c, a colored and long-haired guinea- 

 pig, one of the new F2 varieties. Fig. d, an albino and short-haired 

 guinea-pig, the other new F2 variety. The two other F2 varieties 

 were like the grandparents, Fig. a and b. 



Castle, Heredity and Eugenics. Harvard Press. 



Lethal Genes in Yellow Mice. — There is a type of 

 yellow color characteristic of certain strains of hybrid 

 mice. When these hybrid yellow mice are interbred they 

 should produce 1 pure non-yellow (e.g., black) ; 2 hybrid 

 yellow; 1 pure yellow. As a matter of fact only two 

 yellows (both hybrid) are born for every 1 non-yellow, 

 but the missing pure yellows can be shown to have per- 

 ished before birth. Such genes as this are called lethals 

 and are of considerable importance. Care has to be taken 



