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CARNEGIE INSTITUTION OF WASHINGTON. 



cross-over values; that is, the "distances" are those most likely to be found in 



a new experiment and are, therefore, those that will best serve as a basis for 



prediction. The greatly increased knowledge of double crossing-over in this 



Map of Chromosome III of Drosophila melanogaster* August SO, 1922. 



* Dominants are capitalized. The mutants of greatest importance are indicated by *. 

 mutants of medium value are followed by -\-. 



The 



chromosome has enabled us to convert the recombination percentages into 

 map-distances with a higher degree of accuracy than heretofore. From a 



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Fig. 1. — Base line of figure is the map of chromosome III, simplified by the omission of all 

 except the more important loci. Each curve shows in the height at which it cuts the ordi- 

 nate from a given locus the amount by which crossing-over exceeds recombination. 



study of the cases in which double crossing-over occurs, curves have been 

 constructed (fig. 1) that show the correction on account of multiple crossing- 

 over for any two loci on the map. The curves show the difference between 

 recombination percentage and map-distance for any two loci considered. It 



