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THE AMEBIC AN NATURALIST [Vol. XLIX 



be compared with conditions in the guinea-pig in which yellow 

 spotting is continuous with total black. The essential differ- 

 ences are that in the cat we have a factor for yellow allelomor- 

 phic to a factor for black, that these allelomorphs are sex-linked, 

 and that either alone is sufficient to produce its expected color, 

 but that when one is balanced against the other, as in the tortoise- 

 shell female, other factors governing the relative amounts of the 

 two colors can act and produce continuous variation from yellow 

 to black. 



The three tortoiseshell females from the mating of yellow by 

 yellow may be explained by supposing that the mother was 

 gametically a tortoiseshell plus a sum of yellow extension factors 

 and minus a sum of black extension factors. 



The occurrence of the tabby factor brings in a restriction of 

 the black pigmentation producing yellow stripes. It is there- 

 fore much more difficult to distinguish a tabby from a tabby- 

 tortoiseshell than a black from a tortoiseshell. We have had a 

 few tabby-tortoiseshells that would have been recorded as tab- 

 bies if close examination had not been made. 



Another source of error in records involving the tortoiseshell 

 pattern may be introduced by the occurrence of white spoR. 

 Doncaster makes no mention of these in his paper, so that it is 

 possible that they did not occur in the animals recorded. In 

 what is genetically a tortoiseshell and white cat the incidence 

 of the white spotting may happen to be at just those points 

 which would otherwise be yellow. Thus the occurrence of black 

 and white daughters from yellow males may be explained. It 

 is possible also that the yellow mother of the three tortoiseshell 

 kittens recorded from the mating of yellow by yellow may have 

 been white at points which, if pigmented, would have been black. 

 She would then have been genetically a tortoiseshell and white 

 and some tortoiseshell kittens would have been expected. 



I would suggest as a plausible hypothesis that the rare tor- 

 toiseshell male is genetically a yellow with an extreme of black 

 extension factors or a black with an extreme of yellow extension 

 factors. This hypothesis is rendered more probable by some 

 slight evidence showing that male tortoiseshells breed like 

 yellows. 



There is then no need for assuming in the cat either breaks 

 oogenesis. Phineas W. Whiting 



