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 F{ 1} 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 F{ 2} 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
Since w and g are recessives and therefore disappear when in combination with their respective dominants Y and R the result will be 9 YR (yellow round), 3 Yw (yellow wrinkled), 3 Rg (round green), and 1 gw (green wrinkled) as Mendel actually observed and as all investigators since have confirmed.
Bateson made the discovery that these Mendelian ratios 9: 3: 3: 1 did not always occur when forms differing in two characters were crossed. He found typical and very constant deviations from this ratio in definite cases and these cases he interpreted as being due to "gametic coupling."
These phenomena demonstrate the existence of a complex interrelation between the factorial units. This interrelation is such that certain combinations between factors may be more frequent than others. The circumstances in which this interrelation is developed and takes effect we cannot as yet distinguish, still less can we offer with confidence any positive conception as to the mode in which it is exerted.[206]