MODIFIED Fa RATIOS 117 



ac; the agouti parent AC. Fi is AaCe, a double hetero- 

 zygote. Its gametes consequently should be of four types, 

 viz., AC + Ac + aC + ac, and the F2 zygotes, 9 AC:3 Ac: 

 3 aC:l ac. But only zygotes which contain C will develop 

 a colored coat, hence both 3 Ac and 1 ac will be albinos. The 

 9 AC individuals contain the factors of the wild parent and 

 hence will be agouti; the 3 aC individuals will develop a 

 colored coat since they contain C, but this coat will be non- 

 agouti (a), i. e., they will be like the wild type except for the 

 lack of the agouti factor and so will be black. 



Precisely the same result in Fi and F2 is obtained if a black 

 rodent (rat, mouse, rabbit, or guinea-pig) is crossed with an 

 albino which transmits the agouti factor, as for example an 

 albino whose parents were homozygous for the agouti factor. 

 In this case Fi is agouti by reversion, C being derived from 

 the black parent, A from the albino parent. But Fi is doubly 

 heterozygous, precisely as in the foregoing case, and the F2 

 generation contains only three apparent classes of individuals 

 instead of the usual four for the reason that one of the two 

 differential factors concerned in the cross (viz.. A) is unable 

 to produce a visible effect except in the presence of the 

 other (C). 



Another somewhat similar case involving reversion in Fi 

 with the production of the modified dihybrid ratio, 9:3:4, 

 in F2 is illustrated in Fig. 55. A pale-coated " cream-and- 

 white " rat was crossed with an albino and produced black- 

 and-white young, a reversion to pigmentation of full inten- 

 sity, though white spotting was retained, this being an 

 independent Mendelian character transmitted by both 

 parents. The F2 generation consisted of black-and-white, 

 cream-and-white, and albino individuals in numbers approxi- 

 mating the 9:3:4 ratio. Black-and-white is here the double 

 dominant class, 9; cream-and-white is the single dominant 

 class, 3; and the albinos include three which transmit the 

 dominant character, black-and-white, but which fail to show 

 it because they lack the color factor, and also one which 

 transmits cream-and-white but which fails to show it for the 



