2:-;s 



THE AMERICAN NATURALIST [Vol. XLVII 



given, in the eosin-to-white case, by the fact that the mutant fly 

 was one of 127 obtained from a single pair, all of her brothers 

 and sisters being of the expected classes (half of the females 

 heterozygous for white), as were likewise the flies from six sister 

 pairs. 2 



The presence and absence hypothesis involving a dropping 

 out of whole genes or addition of entirely new ones does not offer 

 as simple an explanation of this case as does the conception that 

 we have here a relatively unstable gene, which does not drop out 

 entirely, but undergoes various changes, that from white to eosin 

 being reversible. 



It should be noted that Shull ('11) has reported a case of 

 what he calls reversible mutation in the sex-determining factor 

 in Lychnis, which is very similar to the above red-eosin-white 

 case. He has adopted a system of triple allelomorphs to explain 

 it, though admitting that complete linkage will also cover the 

 facts. He has also considered the bearing of the case upon the 

 presence and absence hypothesis and upon the nature of muta- 

 tion, reaching conclusions somewhat similar to those given above. 



A. H. Sturtevant 



LITERATURE CITED 



II. Faktorenkoppelung g Zts. ind. Abst.-Vererb.-Lehre, 6, p. 201. 

 Castle, W. E. 1906. Heredity of Coat Characters in Guinea-pigs and 



Rabbits. Carnegie Inst. Wash. publ. 23. 

 (and others). 1909. Studies of Inheritance in Rabbits. Carnegie 



Inst. Wash. publ. 114. 



in Maize. Ann. Kept. Ncbr. Ayr. fiUi., 24, p. 58. 

 Hurst, C. C. 1906. Mendelian Characters in Plants and Animals. Rep. 



Conf. Gen., Boy. Eort. Soc. London, p. 114. 

 de Meijere, J. C. H. 1910. Uber Jacobsons Ziichtungsversuche beziiglich 



des Polymorphisms von Papilio Memnon L. $, und uber die Vererbung 



sekundarer Geschlechtsmerkmale. Zts. ind. Abst.-Vererb.-Lehre, 3. 



p. 161. 



Morgan, T. H. 1912. Further Experiments with Mutations in Eye Color 

 in Drosophila, Jour. Acad. Nat. Sri. Hula., 15, p. 323. 



. 1913. Factors and Unit Characters in Mendelian Heredity. Am. 



Nat., 47, p. 5. 



Punnett, R. C. 1912. Inheritance of Coat-color in Rabbits. Jour. Genet., 

 2, p. 221. 



mutant in red stock, but as there was no other character to serve as a check 



