[Illustration: FIG. 14. Diagram showing inheritance of a sex-linked recessive lethal (“tumor”) factor in _Drosophila melanogaster_. Here, in the center of the diagram, the sex-chromosome that carries the lethal factor is represented by the black rod. A female with the tumor-factor, normal wings and red eyes, in one of her sex-chromosomes and with the factors for yellow wings and eosin eyes in the other is bred in each generation to a male with yellow wings and eosin eyes. In the next generation there are twice as many daughters as sons, since all the sons that carry the black chromosome die. The half of the daughters (_i.e._, those not yellow eosin) that carry the black chromosome repeat the same history. The linkage of yellow and eosin enables one to pick out in each generation those daughters that carry the tumor-factor.] All males that get their single X with this tumor-gene will die; therefore, since no adult males carry it, normal males must be used for mating in each generation. They are mated to females that are heterozygous for the chromosome carrying the tumor genes. Such matings as I have said always give two daughters to one son. But since half the daughters are normal and half carry the gene for tumor it is desirable to be able to pick out the latter from the stock. Therefore we have made use of a trick we call “marking the chromosome,” which means that we use a male whose sex chromosome carries a known gene near the tumor locus. By using this type of male in successive generations we get two types of daughters: one type like their surviving brothers in eye color that do not carry the tumor-gene and the other daughter with normal eyes that carries it. We use only the latter to continue the stock, but we could eliminate the tumor from the stock at once by using the other kind of daughters. Curiously enough the tumor no longer appears in the inbred stock but reappears again on out-breeding. Nevertheless the sex-ratio in the inbred stock continues as before, and since the missing males are those with red eyes we know that the tumor-gene is still present and doing its deadly work—only now the young male larvæ die even before they reach the age at which the tumor is due to appear.