THE METHOD OF CYTOLOGY 481 



X-chromosome is found (cf. pp. 226, 228 and 447). The male 

 sometimes possesses a Y-chromosome which seems to carry^ no 

 genes. At the time of reduction during maturation the eggs formed 

 contain one X-chromosome. Two types of sperm are produced in 

 equal numbers. One kind of sperm has an X-chromosome; the 

 other either has no chromosome homologous with the X, or con- 

 tains a Y-chromosome. The male Drosophila contains both an X 

 and a Y-chromosome. Since the Y carries no genes and never 

 leaves the male hne, it does not influence the course of heredity. 

 In the cross between a homozygous red-eyed female Drosophila 

 and a white-eyed male each egg will contain an X-chromosome 

 carrying a gene for red eye-color, while half the sperm will have 

 an X-chromosome bearing a gene for white eye-color and half will 

 have a Y-chromosome (Fig. 257). Random unions of eggs and 

 sperms will result in red-eyed females and red-eyed males in the 

 Fi generation. The females are red-eyed because the gene for 

 red eye-color is dominant to the gene for white eye-color. They 

 are, however, heterozygous for eye-color, and give rise to two 

 kinds of eggs, in one of which the X-chromosome carries a gene 

 for white eyes, while in the other the X-chromosome carries a gene 

 for red eyes. Half of the sperm produced by the red-eyed males 

 have an X-chromosome with a gene for red eyes, and half have the 

 Y-chromosome. Combinations of the gametes in a cross between 

 a male and a female of the Fi generation result in red-eyod femah\s, 

 red-eyed males, and white-eyed males. In this kind of inheritance 

 the white-eyed characteristic of the male parent is superficially 

 lost in the Fi generation only to repear in one-half the males of 

 the F2 generation. The reciprocal cross, in which a white-eyed 

 female is mated with a red-eyed male, is shown in Fig. 258. If the 

 distribution of the X-chromosomes is followed, the reason for the 

 difference between the offspring from these two crosses should be 

 clear. 



It is to be understood that cases of sex hnkage are not excep- 

 tions to the ]MendeKan principles, but give great weight to the 

 theory of the chromosomes as carriers of the genes. Cytology has 

 furnished the knowledge of the behavior and transmission of 

 chromosomes from one generation to the next. The experimental 

 breeder has been able to explain adequately the results obtained 

 in his breeding experiments, by the assumption that whatever it 

 is that is present in the gametes and determines the appearance 



