Linkage Data. 23 



(i. e., overlaps normal), this value is probably not accurate. When only vesiculated 

 flies are considered the value becomes 1.7 per cent. 



In either case this value, plus the crossveinless-vermilion value of 0.47 unit, con- 

 siderably exceeds the crossveinless-vesiculated value of 1.2 units obtained by Wcin- 

 stein, which suggests that the above sequence of loci may be incorrect. To test this 

 possibility an experiment involving all three genes (crossveinless, vermilion, and 

 vesiculated) together was carried out. 



Vesiculated. 



The results of this experiment (see last paragraph) make it practically certain 

 that the sequence is as given above, i. e., crossveinless-vermilion-vesiculated (see 

 experiment 36, p. 81). The conclusion is warranted in spite of the fact that two of 

 the cross-over classes contain only one fly each, for one of these is critical. It is the 

 one in the crossveinless-vermilion-vesiculated class. The presence of this fly rule* 

 out the order vermilion-crossveinless-vesiculated because it would involve a double 

 cross-over within a "distance" of less than 3 units. Likewise the order crossveinless- 

 vesiculated-vermilion is ruled out by the large number of vermilion flies and of yellow- 

 cross veinless- vesiculated flies, which would also require double cross-overs in this 

 short region. The proper order, therefore, appears to be crossveinless, vermilion, 

 vesiculated, unless an error was made in classification, which seems very unlikely. 



The map locus of vesiculated is calculated by using that of crossveinless as a base 

 and adding the crossveinless-vermilion and vermilion-vesiculated values. These are, 

 respectivelj'^, 0.5, and 1.7, giving a total of 2.2, of a map locus of 26. 



Singed. 



Singed has been tested with vermilion, crossveinless, and yellow in the one direc- 

 tion, and with magenta, forked, triangle, and others in the opposite direction (table 3). 

 Experiment 21, involving yellow, crossveinless, and singed, gives singed a position 

 about 17 units below crossveinless and makes clear the order of the genes. This is 

 verified by experiments 25 and 41, involving yellow, singed, and short in the one 

 case, and vermilion, singed, magenta, and forked in the other. The best available 

 data for locating singed are provided by experiments involving vermilion or cross- 

 veinless on the one side (experiments 6, 21, 34, 41, 48) and magenta forked on the 

 other (experiments 41, 45, 48). The crossveinless-singed value is 15.7, and the 

 vermilion-singed value is 17.9. The former value is probably too low, due to the 

 inclusion of experiment 48, which appears to give low values in the upper part of 

 the map (i. e., the sepia-crossveinless value is only 18.7, and the crossveinless-singed 

 value only 14). On this account, and also because the vermilion-singed value is 

 based on a larger count, the latter is used in constructing the map. It places singed 

 at 43.0. Tests with magenta and forked are considered in the discussion of the latter 

 characters. 



Oblique. 



Oblique was obtained too late, to permit of the data on its linkage relatioca being 

 included in the tables of summaries, hence they will be considered fully here. The 

 approximate location of oblique on the chromosome map was determined by means 

 of two small experiments involving, respectively, magenta, forked, oblique, and short 

 (experiment 49) and sepia, crossveinless, oblique, and short (experiment 50). In the 

 latter only the not-oblique flies are used in the calculation, because oblique interferes 

 with the classification of both sepia and crossveinless. The values obtained in these 

 two experiments indicated that oblique was between crossveinless and forked, so 

 matings were made to test its Hnkage T\ith singed, which has a locus intermediate 

 between these two. The results of the tests with singed are given in experiments 51, 

 52, and 53. The first of these shows that oblique falls very close to singed (2 cross- 



