2o8 MENDELISM 



appear.' Thus grey is epistatic to black, and black 

 is epistatic to chocolate. 



A curious phenomenon appears in the case of yellow 

 mice, which must be briefly mentioned here on account 

 of its bearing upon a subject discussed in the next 

 chapter. Yellow appears to be epistatic to grey as 

 well as to black, but yellow mice, so far as the evidence 

 goes, are always heterozygous. Cue*not's experiment 

 to demonstrate this fact was as follows : 



When YyGGCC is crossed with yyGGCC, equal 

 numbers of yellow and grey offspring are to be ex- 

 pected, since G is hypostatic to Y. In various crosses 

 of this nature Cuenot actually obtained 177 yellows 

 and 178 greys, from which we may deduce that the 

 heterozygote yellow was giving off the expected pro- 

 portion of gametes bearing the yellow character (i.e., 

 50 per cent.). 



When such heterozygous yellows are bred together, 

 the expected result would be as follows : 



YyGGCC x YyGGCC - YYGGCC + 2 YyGGCC + yyGGCC 



3 yellow i grey 



Eighty-one yellow mice were actually obtained in 

 this way. Among them some twenty-seven would 

 naturally be expected to be pure dominant, and to give 

 yellow only when crossed with black or grey indi- 

 viduals. To Cuenot's astonishment, he found on making 

 the necessary crosses that every one of these eighty-one 

 yellows gave some black or grey among its offspring ; 

 not one of them was a pure homozygous yellow. 



The facts are explicable in one of two ways. On the 



