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] [206] Bateson, W., _loc. cit._, p. 157. Morgan has given an ingenious explanation of these deviations on the basis of the chromosome theory of Mendelian heredity. He assumes that they occur in those cases where the two or more characters are contained in the same chromosome. In that case the two factors lying in the same chromosome should generally be found together. Such was the case for instance in the experiments with flies having red eyes and yellow body colour _versus_ white eyes and grey body colour, the character for white eyes and yellow body being located in the X chromosome (see preceding chapter), or in the experiments on _Abraxas_. These phenomena are called linkage, and the numerical results of linkage were given in the preceding chapter in connection with the crossing of sex-linked characters.