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[Transcriber's note: displayed in the text as YR over gw, i.e. like a fraction but without a dividing bar]. 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, YRYR YRgw YRgR YRYw gwYR gwgw gwgR gwYw gRYR gRgw gRgR gRYw YwYR Ywgw YwgR YwYw