208 MENDELISM 
A curious and not fully explained 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. Cuénot’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 Cuénot 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 Cuénot’s astonishment, he found on making 
the necessary crosses that every one of these eighty- 
one yellows gave some black or grey among its off- 
spring; not one of them was a pure homozygous 
yellow. 
