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 =X=X red-eyed female =X= Y =X=Y 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 =X=X 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 =X=Y 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) =X==X=, (2) =X=X, (3) =X=Y, (4) XY, all four types in equal numbers.