116 



GENETICS AND EUGENICS 



out the factor (E) necessary to make it visible, and so will be 

 red; and ea contains neither the factor for agouti nor that 

 for black, hence will also be red. Accordingly the expected 

 Fo distribution is nine agouti, three black, four red, the ratio 

 observed. This is a very common modification of the F2 



Fig. 54. Checkerboard to explain the modified dihybrid F2 ratio, 

 9 : 3 : 4, as observed when black guinea-pigs are crossed with red ones 

 which transmit the agouti factor (A). 



dihybrid ratio and owes its production to the fact that two 

 independent Mendelian factors are involved one of which pro- 

 duces no visible effect except in the presence of the other. 



Another example of this same modified dihybrid ratio 

 (9:3:4) is obtained by crossing an albino rodent (rat, mouse, 

 rabbit or guinea-pig) derived from a black race, with a wild 

 (agouti) individual. Fi consists of agoutis, like the wild 

 parent, but F2 contains agoutis, blacks, and albinos in the 

 proportions, nine agouti, three black, four albino. The ex- 

 planation is as follows. The albino parent differs from the 

 wild agouti parent as regards two factors, viz., the color 

 factor (C) and the agouti factor (A). The albino parent is 



