[Illustration: FIG. 35. Diagram showing a cross between a white eyed male and a red eyed female of the fruit fly. Sex linked inheritance.] The reciprocal cross (fig. 36) is also interesting. If a white eyed female is bred to a red eyed male, all of the daughters have red eyes and all of the sons have white eyes. We call this criss-cross inheritance. If these offspring are inbred, they produce equal numbers of red eyed and white eyed females and equal numbers of red eyed and white eyed males. The ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in entire accord with Mendel's principle of segregation. [Illustration: FIG. 36. Diagram illustrating a cross between a red eyed male and white eyed female of the fruit fly (reciprocal cross of that shown in Fig. 35).] It has been shown by Sturtevant that in a wild species of Drosophila, viz., D. repleta, two varieties of individuals exist, in one of which the thorax has large splotches and in the other type smaller splotches (fig. 37). The factors that differentiate these varieties are sex linked. [Illustration: FIG. 37. Two types of markings on thorax of Drosophila repleta, both found "wild". They show sex linked inheritance.] Certain types of color blindness (fig. 38) and certain other abnormal conditions in man such as haemophilia, are transmitted as sex linked characters. [Illustration: FIG. 38, A. Diagram illustrating inheritance of color blindness in man; the iris of the color-blind eye is here black.] [Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.] In domestic fowls sex linked inheritance has been found as the characteristic method of transmission for at least as many as six characters, but here the relation of the sexes is in a sense reversed. For instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock (fig. 39), the offspring are all barred. If these are inbred half of the daughters are black and half are barred; all of the sons are barred. The grandmother has transmitted her color to half of her granddaughters but to none of her grandsons. [Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown here in a cross between barred Plymouth Rock male and black Langshan female.] [Illustration: FIG. 40. Reciprocal of Fig. 39.] In the reciprocal cross (fig. 40) black cock by barred hen, the daughters are black and the sons barred--criss-cross inheritance. These inbred give black hens and black cocks, barred hens and barred cocks.