The explanation of this mode of transmission became clear when it was found that the cause of the death of half of the males was a particular factor that had as definite a locus in the X chromosome as have other sex-linked factors (Morgan, 1912_e_). Morgan mated females (from the stock sent to him by Miss Rawls) to white-eyed males. Half of the females, as expected, gave 2 : 1 sex-ratios, and daughters from these were again mated to white males. Here once more half of the daughters gave 2 : 1 sex-ratios, but in such cases the sons were nearly all white-eyed and only rarely a red-eyed son appeared, when under ordinary circumstances there should be just as many red sons as white sons. The total output for 11 such females was as follows (Morgan, 1914_b_): white [female], 457; red [female], 433; white [male], 370; red [male], 2. It is evident from these data that there must be present in the sex-chromosome a gen that causes the death of every male that receives this chromosome, and that this lethal factor lies very close to the factor for white eyes. The linkage of this lethal (now called lethal 1) to various other sex-linked gens was determined (Morgan 1914_b_), and is summarized in table 5. On the basis of these data it is found that the gen lethal 1 lies 0.4 unit to the left of white, or at 0.7. TABLE 5.--_Summary of linkage data upon lethal 1, from Morgan, 1914b, pp. 81-92._ +------------------------+---------+--------+-------------+ | Gens. | Total. | Cross- | Cross-over | | | | overs. | values. | +------------------------+---------+--------+-------------+ | | | | | | Yellow lethal 1 | 131 | 1 | 0.8 | | Yellow miniature | 131 | 45 | 34.4 | | Lethal 1 white | 1,763 | 7 | 0.4 | | Lethal 1 miniature | 814 | 323 | 39.7 | | White miniature | 994 | 397 | 39.9 | | | | | | +------------------------+---------+--------+-------------+ LETHAL 1a.