The result would be expressed by the formula _AA:_ _Aa:_ _aa_ which is the same as that given for any character in a Mendelian case. Thus the phenomena of germ cell division and of heredity are seen to have the same essential features viz., purity of units (chromosomes, characters) and the independent transmission of the same; while as a corollary it follows in each case that each of the two antagonistic units (chromosomes, characters) is contained by exactly half the gametes produced. It is obvious that Sutton by this idea did for heredity in general what McClung had done for sex determination or sex heredity, that is, he showed that the numerical results obtained in Mendelian heredity can be accounted for on the basis that factors for hereditary characters are carried by definite chromosomes. The cytological basis of sex determination becomes only a special case of the cytological basis of Mendelian heredity. In the examples quoted the plants giving rise to violet and to white flowers are homozygous for the colour of flower having the chromosome constitution _AA_ and _aa_ respectively; while the plants with pale violet flowers are heterozygous, having the chromosome constitution _Aa_ in their nuclei. The former give rise to identical sex cells _A_ and _A_ or _a_ and _a_; while the heterozygous plants give rise to different sex cells _A_ and _a_. From this point of view in _Drosophila_ (and very probably also in man) the female is homozygous for sex having in all its cells the critical chromosome constitution XX and giving rise to one type of eggs only, each with one X chromosome; while the male in these forms is heterozygous for sex having in all its cells the chromosome constitution XY and forming two different types of spermatozoa in equal numbers X and Y. In _Abraxas_ and in the fowl the female is heterozygous for sex and the male homozygous. 3. If the chromosomes are the vehicle for Mendelian heredity it should be possible to show that the various hereditary characters which follow Mendel’s law must be distributed over the various chromosomes; and it should be possible to find out which characters are contained in the same chromosome. It has already been stated that sex-linked heredity is intelligible on the assumption that the X chromosome carries the sex-linked characters. T. H. Morgan and his pupils have shown with the greatest degree of probability that corresponding linkages occur in the other chromosomes and that there are in _Drosophila_ exactly as many groups of linkage as there are different chromosomes, namely four.[205] [205] Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., _The Mechanism of Mendelian Heredity_. New York, 1915.