The main fact is that the female cells have the chromatin composition XX, the male cells the composition XY, where Y is apparently qualitatively different and often, but not necessarily, smaller than X, or entirely lacking.
It may be mentioned in passing that indirect evidence exists indicating that in man there are also two kinds of spermatozoa and one kind of egg, and that sex depends on whether a male determining or a female determining spermatozoön enters the egg.
2. This mode of sex determination holds only for those animals in which there is one type of egg and two types of spermatozoa. Experimental evidence furnished first by Doncaster in 1908 on a moth, Abraxas, indicated that a number of other forms exists in which matters are reversed, inasmuch as there are two types of eggs and one type of spermatozoa. This condition of affairs exists not only in the moth Abraxas, but also in the fowl as shown by Pearl. In these forms it is assumed that all the spermatozoa have one sex chromosome X, while there are two types of eggs, one possessing the sex chromosome X, the other possessing Y. When a spermatozoön enters an egg with an X chromosome, the egg will give rise to a male, while if it enters a Y egg, a female will arise. The evidence pointing toward this result is chiefly contained in experiments on sex-limited or more correctly sex-linked heredity; i. e., a form of heredity which follows the sex in a peculiar way. Thus colour-blindness is a case of sex-linked inheritance, since this abnormality appears overwhelmingly in the male offspring of a colour-blind person. Doncaster crossed two varieties of Abraxas differing in one character which was sex-linked, and the results of his crossings indicated that in this form there are two types of eggs and one type of spermatozoa.[184]
These observations on sex-linked heredity confirm the idea that the sex chromosomes determine the sex. The most extensive and conclusive experiments along this line are those by Morgan on the fruit fly Drosophila. In this form there are two kinds of spermatozoa and one kind of eggs; the egg has one X chromosome, while one-half of the spermatozoa has an X the other a Y chromosome; the entrance of the latter into an egg gives rise to a male, of the former to a female.
While the eyes of the wild fruit fly Drosophila ampelophila are red, Morgan[185] noticed in one of his cultures a male that had white eyes. This white-eyed male was mated to a red-eyed female. The offspring, the F1 generation, were all red eyed, males as well as females. These were inbred and now gave in the F2 generation the following three types of offspring:
| (1) 50 per cent. females, all with red eyes. | ||
| (2) 50 per cent. males |  | 25 per cent. with red eyes. |
| 25 per cent. with white eyes. | ||
The character white eye was therefore transmitted only to half the grandsons; it was a sex-linked character. It is known from a study of the pedigrees of colour-blind individuals that if the corresponding experiment had been carried out with them, instead of with white-eyed flies, the same proportions of normal and colour-blind would have been found: namely, normal colour vision in the F1 generation, in both males and females, and half of the males of the F2 generation colour-blind, the other half and all the females with normal vision. Of course, in man, intermarriage between two different F1 strains would have been required in place of the inbreeding of the F1 generation, which took place in Morgan’s experiments. Morgan interprets his experiments as follows. The normal red-eyed Drosophila has one kind of eggs, each possessing one X chromosome. This X chromosome has also the factor for the development of red-eye pigment. The white-eyed male has two kinds of spermatozoa, one with an X chromosome, the other with a Y chromosome, both lacking the factor for red-eye pigment. If we designate the X chromosome with the factor for red-eye pigment by X and the X and Y chromosomes lacking the factor for redness with X and Y the following combinations must result if we cross a normal red-eyed female with a white-eyed male:
| Eggs | Sperm | Result |
| X | X | XX red-eyed female |
| X | Y | XY red-eyed male |
It is obvious that all the offspring of the first generation (the F1 generation) must be red eyed, since all the eggs have one X chromosome with the factor for red. According to the results obtained from cytological studies which will be explained in the next chapter, the females with the chromatin constitution XX will form two types of eggs in equal numbers: namely, eggs with an X and eggs with an X, i. e., all eggs have one X chromosome, but in fifty per cent. of the eggs the X has the factor for red, in fifty per cent. this factor is lacking (X). The males having the chromosome constitution XY form two types of spermatozoa, one with an X possessing the factor for red pigment and one, the Y chromosomes, lacking this factor. If inbred the next F2 generation will give rise to the following four types of offspring: (1) XX, (2) XX, (3) XY, (4) XY, all four types in equal numbers.