If, then, as these and other results to be described point to the chromosomes as the bearers of the Mendelian factors, and if, as will be shown presently, these factors have a definite location in the chromosomes it is clear that the location of the factors in the chromosomes bears no spatial relation to the location of the parts of the body to each other. LOCALIZATION OF FACTORS IN THE CHROMOSOMES _The Evidence from Sex Linked Inheritance_ When we follow the history of pairs of chromosomes we find that their distribution in successive generations is paralleled by the inheritance of Mendelian characters. This is best shown in the sex chromosomes (fig. 57). In the female there are two of these chromosomes that we call the X chromosomes; in the male there are also two but one differs from those of the female in its shape, and in the fact that it carries none of the normal allelomorphs of the mutant factors. It is called the Y chromosome. The course followed by the sex chromosomes and that by the characters in the case of sex linked inheritance are shown in the next diagram of Drosophila illustrating a cross between a white eyed male and a red eyed female. [Illustration: FIG. 57. Scheme of sex determination in Drosophila type. Each _mature_ egg contains one X, each mature sperm contains one X, or a Y chromosome. Chance union of any egg with any sperm will give either XX (female) or XY (male).] [Illustration: FIG. 58. Cross between white eyed male of D. ampelophila and red eyed female. The sex chromosomes are indicated by the rods. A black rod indicates that the chromosome carries the factor for red; the open chromosome the factor for white eye color.] The first of these represents a cross between a white eyed male and a red eyed female (fig. 58, top row). The X chromosome in the male is represented by an open bar, the Y chromosome is bent. In the female the two X chromosomes are black. Each egg of such a female will contain one "black" X after the polar bodies have been thrown off. In the male there will be two classes of sperm--the female-producing, carrying the (open) X, and the male-producing, carrying the Y chromosome. Any egg fertilized by an X bearing sperm will produce a female that will have red eyes because the X (black) chromosome it gets from the mother carries the dominant factor for red. Any egg fertilized by a Y-bearing sperm will produce a male that will also have red eyes because he gets his (black) X chromosome from his mother.