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, CV. Theoretically one would 

 expect to obtain, Cuenot thinks, such a race by crossing two 

 similar, heterozygote, yellow mice. Thus — 



CYCG X CYCG = ?^^^ + ' ^^^^ ^^^^ 



3 yellow i gray 



One third of the yellows should be pure yellow with gametes 

 all CF's. They should breed true to their color. Of 81 yellow 

 mice obtained by such combinations, not one proved to be pure 

 CF's. In connection with this result, Cuenot 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 7,1^.1, per cent grays; but neither 

 does this occur,^ and the proportions are more nearly 75 per 

 cent to 25 per cent. Therefore Cuenot offers the following ex- 

 planation: Since his yellow mice were never homozygotes, it 

 follows that the combination of CF with CY can never occur 

 in fertilization, although Cuenot thinks that other results 

 show that a disjunction of CF from the other colored charac- 

 ters takes place. This means that these gametes, CY, can 

 never meet to give the zygotes having the formulae CYCY, 



^ Nevertheless yellow must have been latent, as in the case of albinos that 

 contain a latent color and still breed true. 



^ 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. 



