144 MUTATION. 



the two species have both genes, and as a result their skin is 

 pigmented, but very much lighter than that of the pure Silky. 

 If, however, the Silky is mated to light-skinned fowls lacking 

 the "lightening" gene, the hybrids are as intensely pigmented 

 as the real Silkies. 



Now it would seem, as if it would always be easy to disting- 

 uish between cases in which an intermediate hybrid were 

 half-way between his parents in some character because he 

 inherited from each a gene, which genes counteracted each 

 others effects, or whether the intermediate condition of the 

 character was due to the fact, that one single gene if transmit- 

 ted only in one gamete were less effective than in double dose. 

 For the children of such a hybrid would in one case show a 

 bi-factorial segregation, 9:3:3:1 and the off-spring in the 

 other case would segregate into a i : 2 : i ratio. But we must 

 remember that in a few instances we get an apparent mono- 

 factorial segregation, i : 2 : i, in those instances which Bate- 

 son has designated by the name "spurious allelomorphism". 

 An individual, impure for two genes, sometimes produces only 

 two kinds of germ-cells, instead of the ordinary four, AB, Ab, 

 aB and ab, namely only germ-cells containing either one or the 

 other gene, and none of the other categories. Now such in- 

 stances are easily mistaken for cases, in which heterozygotes 

 show the action of a gene, which they have inherited in one 

 germ-cell only to about half the extent of what it amounts to in 

 homozygotes. One of us has investigated a case in mice, in 

 which such a spurious allelomorphism existed between the 

 gene which agouti animals have more than black ones and that 

 which pigmented animals have more than albinos. Agouti 

 hybrids in this series, produced by mating an albino to a black, 

 did not produce nine agouti to three black and four albinos, of 

 which agou is there would be several kinds, including homo- 

 zygotes, and of which albinos there would be two kinds, but 

 two agouti heterozygotes, to one black and one albino homo- 

 zygotes. If the two factors concerned had not been known be- 

 forehand, in other series of experiments, in which they were 



