Now this is exactly the ratio of Mendelian heredity in the F2 genera­tion. The plant with the chromo­some constitu­tion AA will form violet flowers, those with the chromo­some constitu­tion Aa will form pale violet flowers, and those with the chromo­some constitu­tion aa will form white flowers.

To quote Sutton’s words:

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]

Mendel had found that when he crossed two species of peas differing in regard to two pairs of characters, he obtained in the F2 genera­tion results which he calculated on the assump­tion that the segrega­tion of the two pairs of characters in the sex cells of the hybrids took place independently of each other. To illustrate by an example: When crossing a yellow round pea with a green wrinkled variety in which the characters round and yellow are dominant, green and wrinkled recessive, all the hybrids of the F1 genera­tion had the characters round and yellow. When these were inbred the F2 genera­tion produced four types of seed in the ratio 9: 3: 3: 1, namely:

(1) yellow round (315 seeds)
(2) yellow wrinkled (101 seeds)
(3) green round (108 seeds)
(4) green wrinkled (32 seeds)

The explana­tion according to Mendel’s theory is as follows: Since the segrega­tion of each pair of characters occurs independently, there must be 3 yellow to 1 green and also 3 round to 1 wrinkled in the F2 genera­tion. The yellow will, therefore, be round and wrinkled in the ratio of 3:1, which will give 9 yellow round to 3 yellow wrinkled. The green will also be round and wrinkled in the ratio of 3:1, which will give 3 green round to 1 green wrinkled, which is the ratio of 9: 3: 3: 1 found by Mendel.

On the basis of the chromo­some theory the following explana­tion could be given of this numerical rela­tion. The peas with yellow round seeds have sex cells with a factor for both yellow and for round in two different chromo­somes; these two different chromo­somes we will designate with Y and R. The peas with green and wrinkled seeds will have in their sex cells factors for these characters in two homologous chromo­somes g and w, where g is the homologue of Y and w of R. The cells of the hybrids of the F1 genera­tion will have the chromo­some constitu­tion Yg Rw, where Y and g and R and w are homologous chromo­somes which will lie alongside each other YRgw. In the forma­tion of sex cells a reduc­tion of these four chromo­somes to two takes place whereby, according to the theory of Sutton, the following two types of separa­tion can take place: YR and gw, or gR and Yw. (A separa­tion into Yg and Rw is impossible since the division takes place only between homologous chromo­somes.) Hence there will be four types of eggs, YR, gw, gR, and Yw and the same four types of pollen cells. The F2 genera­tion will produce the sixteen possible combina­tions in equal numbers: namely,