The reciprocal cross was tried many times, but is impossible, owing to the sterility of the females. Since the fused females are sterile to fused males, the stock is kept up by breeding heterozygous females to fused males.
By means of the following experiments the position of fused in the X chromosome was determined. A preliminary test was made by mating with eosin, whose factor lies near the left end of the X chromosome series.
Fused (red-eyed) males mated to eosin (not-fused) females gave wild-type daughters and eosin sons, which inbred gave the classes shown in table 33.
TABLE 33.--_P_1 eosin [female][female] × fused [male][male]. F_1 wild-type [female][female] × F_1 eosin [male][male]._
+----------+--------+-----------------+----------------+-------+--------+ | | | Non-cross-over | Cross-over | | | | | | [male][male]. | [male][male]. | Total | Cross- | |Reference.|Females.+--------+--------+--------+-------+ males.| over | | | | Eosin. | Fused. | Eosin | Wild- | | value. | | | | | | fused. | type. | | | +----------+--------+--------+--------+--------+-------+-------+--------+ | 56I | 496 | 131 | 113 | 82 | 104 | 430 | 43 | +----------+--------+--------+--------+--------+-------+-------+--------+
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.