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THE AMERICAN NATURALIST [Vol. XLIX 



intense but numerous red-eye young. The intense ani- 

 mals and the albinos used were of the same stock and 

 hence could differ consistently only as regards albinism. 

 Clearly red-eye is not inherited independently of albinism. 

 That the apparent reversal of dominance is not due 

 merely to the presence of recessive albinism is shown by 

 the fact that F x red-eyes crossed inter se produced 89 red- 

 eyes, 36 albinos, but no intense young. Among these F 2 

 red-eyes, some were demonstrated to be free from re- 

 cessive albinism. This complete inability of albinos to 

 transmit the intense allelomorph of red-eye in crosses 

 with the latter can only be interpreted in one way, aside 

 from linkage hypotheses. The intense allelomorph of 

 red-eye must also be an allelomorph of albinism. Thus 

 red-eye must either be an allelomorph of albinism or be 

 albinism itself, genetically, plus a modifying factor. The 

 latter rather improbable hypothesis has been definitely 

 eliminated. The pure South American stock under this 

 hypothesis must be homozygous for all such modifying 

 factors, since no albinos have appeared among them. The 

 hypothetical modifying factor must be a dominant unit 

 factor to account for the results given for Fj and F 2 . 



Some pure South American intense animals were crossed 

 with stock albinos. They produced F 1 intense young, 

 which must (under the hypothesis) be heterozygous for 

 albinism and for the modifying factor. These Fj in- 

 tense young were back-crossed with stock albinos. There 

 should be both red-eyes and albinos among the young, 

 25 per cent, of each, if red-eye is albinism associated with 

 a modifier. But no red-eyes actually appeared. There 

 were 14 intense and 25 albinos. The chance that no red- 

 eyes would appear in 39 young is (3/4), 39 or .000,001. 

 Thus the hypothesis that red-eye is albinism plus a modi- 

 fier may be dismissed. 



On the view that red-eye is a dominant allelomorph of 

 albinism, the results above are easily explained. Red- 

 eye by albino or by red-eye should give only red-eyes and 

 albinos. The Fjl intense, which must carry recessive al- 



