The factor for yellow body-color lies at one end of the known series of sex-linked gens. As already stated, we speak of this end as the left end of the diagram, and yellow as the zero in locating factors. When yellow (not-sable) females were mated to (not-yellow) sable males they gave wild-type (gray) daughters and yellow sons. These inbred gave in F_2 two classes of females, namely, yellow and gray, and four classes of males, namely, yellow and sable (non-cross-overs), wild type and the double recessive yellow sable (cross-overs). From off-spring (F_3) of the F_2 yellow sable males by F_2 yellow females, pure stock of the double recessive yellow sable was made up and used in the crosses to test linkage. In color the yellow sable is quite similar to yellow black, that is, a rich brown with a very dark brown trident pattern on the thorax. Yellow sable is easier to distinguish from yellow than is yellow black, even when the flies have not yet acquired their adult body-color. Yellow sable males were bred to wild females and F_1 consisted of wild-type males and females. These inbred gave the results shown in table 8. {36} TABLE 8.--_P_1 wild [female] [female] x yellow sable [male] [male]. F_1 wild-type [female] [female] x F_1 wild-type [male] [male]._ +-----------+---------+--------------+--------------+-------+-----------+ | | |Non-cross-over| Cross-over | | | | | Wild- | [male]. | [male]. | | | | Reference.| type +-------+------+-------+------+ Total |Cross-over | | |[female].| Yellow| Wild-| | | males.| value. | | | | sable.| type.|Yellow.|Sable.| | | +-----------+---------+-------+------+-------+------+-------+-----------+ | | | | | | | | | | 44 I | 292 | 110 | 43 | 75 | 36 | 264 | 42 | | 45 I | 384 | 104 | 58 | 71 | 60 | 293 | 45 | | +---------+--------------+-------+------+-------+-----------+ | Total | 676 | 214 | 101 | 146 | 96 | 557 | 43 | | | | | | | | | | +-----------+---------+-------+------+-------+------+-------+-----------+ Some of the F_1 females were back-crossed to yellow sable males and gave the data for table 9. TABLE 9.--_P_1 wild-type [female] [female] x yellow sable [male] [male]. B. C. F_1 wild-type [female] x yellow sable [male] [male]._