The data give 43 per cent of crossing-over, which places fused far to the right or to the left of eosin. The latter position is improbable, since eosin already lies very near the extreme left end of the known series. Therefore, since 43 per cent would place the factor nearly at the right end of the series, the next step was to test its relation to a factor like bar that lies at the right end of the chromosome. By mating to bar alone we could only get the linkage to bar without discovering on which side of bar the new factor lies, but by mating to a fly that carries still another sex-linked factor, known to lie to the left of bar, the information gained should show the relative order of the factors involved. Furthermore, since, by making a back-cross, both males and females give the same kind of data (and need not be separated), the experiment was made in this way. In order to have material for such an experiment double mutant stocks of vermilion fused and also of bar fused were made up.

Fig. D.—a, normal wing; b and c, fused wings. c shows a typical fused wing. The most striking feature is the closure of the cell between the third and fourth longitudinal veins with the elimination of the cross-vein; the veins at the base of the wing differ from those in the normal shown in a. b shows the normal position in which the fused wings are held. The fusion of the veins in b is unusually complete.

LINKAGE OF VERMILION, BAR, AND FUSED.

Males from the stock of (red) bar fused were mated to vermilion (not-bar, not-fused) females, and produced bar females and vermilion males. The bar F1 daughters were back-crossed to vermilion fused males and produced the classes of offspring shown in table 34.

Table 34.—P1 vermilion ♀ ♀ × bar fused ♂ ♂. B. C. F1 bar ♀ × vermilion fused ♂ ♂.

Reference.

Total. Cross-over values.
Ver-
milion.		 Bar
fused.		 Ver-
milion
bar
fused.		 Wild-
type.		 Ver-
milion
fused.		 Bar. Ver-
milion
bar.		 Fused. Ver-
milion
bar.		 Bar
fused.		 Ver-
milion
fused.
140 I 137 130 35 40 5 8 .. .. 355 21 4 25
141 I 144 137 38 41 4 2 .. .. 366 22 2 23
142 I 153 120 43 58 6 7 1 .. 388 26 4 29
143 I 153 92 44 41 3 7 3 1 344 26 4 28
145 I 69 62 29 19 1 .. 1 .. 181 27 1 27
146 I 96 103 30 34 7 3 .. .. 273 23 4 26
156 I 62 45 25 27 1 4 .. .. 164 32 3 35
157 I 93 57 11 31 2 2 .. 2 198 22 3 23
Total. 907 746 255 291 29 33 5 3 2,269 24 3 27

The data show that the factor for fused lies about 3 units to the right of bar. This is the furthest point yet obtained to the right. The reasons for locating fused to the right of bar are that, if it occupies such a position, then the double cross-over classes (which are expected to be the smallest classes) should be vermilion bar and fused, and these are, in fact, the smallest classes. The order of factors is, then, vermilion, bar, fused. This order is confirmed by the result that the number of cross-overs between fused and vermilion is greater than that between bar and vermilion.