OF MUTANT CHARACTERS. 153 



chromosome being a cross-over. By breeding together some of these 

 F3 blacks, in F4 black vestigial flies were obtained from which a stock 

 was made that is still running. 



LOCUS OF VESTIGIAL. 



By aid of this stock of black-vestigial it was possible to make back- 

 cross tests of the amount of crossing-over. When these experiments 

 were carried out by Morgan it was found that there was about 22 i)or 

 cent of crossing-over in the female, but none whatever in the male 

 (Morgan, 1912). The principle of no-crossing-over in the male, 

 first clearly demonstrated in the above back-cross, has been found 

 to apply to all cases in both the second and third chromosomes of 

 Drosophila. 



From the early data of Morgan it was known that the two loci, 

 black and vestigial, were about 22 units from each other. Several 

 errors have been found in the data for crossing-over as first presented 

 (Morgan, 1912). These data, as corrected and considerably added to 

 (Morgan, 1914), show that the locus of vestigial is about 18 rather than 

 22 units from black. 



The mapping of vestigia^l in relation to the other second-chromosome 

 genes was carried out by Bridges (through the use of purple as a sec- 

 ondary base) and by Sturtevant (through use of curved as a base of 

 reference) . Relatively large amounts of data involving the relation of 

 vestigial and other second-chromosome mutants was soon secured. 

 The most useful of these determina,tions have been the various purple- 

 vestigial data, for purple is the base of reference for vestigial. Table 17 

 gives a summary of this early data and of all that have since become 

 available. 



VALUATION OF VESTIGIAL. 



Vestigial, while it is not strictly of first rank in usefulness, approaches 

 very close to this standard. In ease and quickness of separation from 

 the wild-type it is unsurpassed. Its position in the chromosome is 

 one which is important and convenient. Enough work has been done 

 using vestigial as material, so that in undertaking fresh work one can 

 count on a sound basis for comparison. Its viabiUty is good under 

 good cultural conditions. The above are the points in its favor; its 

 disadvantages are that it masks all other wing-characters, so that its 

 use in an experiment materially reduces the avaihibility of other \\ing- 

 characters, some of which, such as curved, are themselves of first rank. 

 Its viabilty is apt to be poor unless careful and experienced attention 

 is given to cultural conditions, and its lateness of hatching and the diffi- 

 culty of getting the vestigials out of the culture bottle also tend to 

 give aberrant ratios to the unwary. 



