Experimental Hybridizing 95 
heterozygote, it should give, when crossed with a pure race, half 
yellow and half the other color (as stated above), according 
to the formula: CYCG x CGCG = CYCG (yellow) + CGCG 
(gray). The gray offspring should be pure and never produce 
any yellow. Such was found to be the case. Thus of 355 
young there were 177 yellow and 178 gray or black. The 
gray or black do not include the recessive yellow —at least 
yellow mice never appear in their progeny.’ 
The remarkable fact, referred to above, is that it is impossible 
to obtain a pure yellow race, CY. Theoretically one would 
expect to obtain, Cuénot thinks, such a race by crossing two 
similar, heterozygote, yellow mice. Thus — 
CYCY+2CGCY CGCG 
CYCG x CYCG= 
3 yellow I gray 
One third of the yellows should be pure yellow with gametes 
all CY’s. They should breed true to their color. Of 81 yellow 
mice obtained by such combinations, not one proved to be pure 
CY’s. In connection with this result, Cuénot finds that there 
are always fewer yellows than expected on the last formula, 
which gives 75 per cent yellow to 25 per cent grays. If we 
assume that this is due to the absence of CYCY, then we should 
expect 66.6 per cent yellows to 33.3 per cent grays; but neither 
does this occur,’ and the proportions are more nearly 75 per 
cent to 25 percent. Therefore Cuénot offers the following ex- 
planation: Since his yellow mice were never homozygotes, it 
follows that the combination of CY with CY can never occur 
in fertilization, although Cuénot thinks that other results 
show that a disjunction of CY from the other colored charac- 
ters takes place. This means that these gametes, CY, can 
never meet to give the zygotes having the formule CYCY, 
1 Nevertheless yellow must have been latent, as in the case of albinos that 
contain a latent color and still breed true. 
2 Professor E. B. Wilson has pointed out that even if the CY sperm never 
fertilizes the CY eggs, the expectation would still be 75 :: 25, because the CY 
eggs would be fertilized by other sperm. 
