There is a class of cases of inheritance, due to the XY chromosomes, that is called sex linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance. If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. The grandfather has transmitted his peculiarity to half of his grandsons, but to none of his granddaughters.
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.
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