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Seven mutant genes of Drosophila simulans have been shown to be allelomorphic to previously known mutant genes of D. melanogaster. Five of these lie in the X-chromosome, and a study of their linkage relations was shown to indicate that the sequence of the five loci concerned is the same in both species, and that the percentages of crossing over in comparable regions, while not indentical, is still not very different. The other two allelomorphic mutant genes, scarlet and peach, lie about 3 units apart in the third chromosome of melanogaster; in simulans they lie in the same chromosome (which is thus identified as the third one), but they were found to be at least 45 units apart.
A. H. Sturtevant (1921) studied this question.