70 ' PHYSICAL BASIS OF HEREDITY 



of Eose, and the Rose comb the normal recessive allelo- 

 morph of Pea : but the result is not an atavistic normal 

 comb, but a Walnut produced by the action of both domi- 

 nants that are here the mutant characters. 



An important class of factors that are known as 

 diluters or intensifiers are often met with in genetic 

 work. For instance, a black mouse pure for a certain 

 "diluting" factor has a "blue" color (just as a black 

 mouse pure for albino factors is white). Such a blue 

 mouse crossed to black gives F^ black mice, and in Fg 

 three blacks to one blue. A two-factor cross results when 

 a blue mouse is bred to a " chocolate ' ' (=black cinnamon) 

 mouse. The F^ will be black, the F2 will be 9 black, 3 blue, 

 8 chocolate, 1 "silver-fawn" (dilute black cinnamon). 

 In this case, the same factor that changes black to blue also 

 changes chocolate to silver-fawn. If the diluter had 

 been a specific one affecting black only, then F^ from the 

 above cross would have been 9 black, 3 blue, 4 chocolate. 

 Such a case is found in the vinegar fly, in which the diluter 

 affects only a recessive factor — eosin. This specific 

 diluter for eosin is called ' ' whiting. ' ' It gives the follow- 

 ing results : A red-eyed female homozygous for whiting 

 is indistinguishable from the ordinary wild type. If a 

 female of this kind is crossed to an eosin male the off- 

 spring (JPi) are red eyed. If they are inbred they give 12 

 red-eyed flies, 3 eosin, 1 eosin-whiting which is colorless. 



Another modification of the 9:3:3:1 ratio appears 

 when the last three classes are superficially alike. For ex- 

 ample, Bateson and Punnett crossed two white flowering 

 varieties of sweet peas. The F^ had purple flowers, which, 

 inbred, gave 9 purple and 7 whites. Here there are two 

 different recessive factors which in homozygous condition 

 give white, ww and aa; each has a normal dominant allelo- 

 morph in the other white, AA and WW. The two white 

 parents are then wtvAA and aaWW. The Fj individuals 



