But in the phenomena of hybridism, we have seen the results of another fact which determines Mendel's third law; the Law of the Independence of Characteristics.
That is, that while the original progenitors had angular seed and green cotyledons, and round seed and yellow cotyledons, certain hybrid plants inherited the round seed of the one and the green colour of the other; or the angular seed of the one and the yellow colour of the other. In the same way, it may happen, for example, that the colour of one plant may combine with the height of another, etc. That is, that each separate characteristic of the progenitor is independent and may combine with the characteristics of the other progenitor—even to the point of separating the colour from the form, as in the case cited.
What we find in hybrids, then, is not a separation into two types of generative cells, considered as united and complex entities; but every separate germ cell may break up into as many different potentialities as there are separate characteristics in the individual; and that, too, not only as regards the separate minute parts of the individual body, but, within the same organ, as regards the shape, colour, character of the surface, etc.
Such phenomena of Mendelism cannot as yet be generalised; yet it has already been established by a host of experiments that a great number of characteristics obey the laws of Mendel, such, for example, as the character of the hair or plumage; the gradations of colour, the abundance or absence of hair; physical malformations, such as cerebral hernia in poultry; the character of locomotion, as in the jumping mice: and even normal physiological attributes connected with the epoch of maturity in certain plants.
But the manner in which the dominant character asserts itself is not always uniform. There are times when a fusion of antagonistic characters takes place. Thus, for example, when two varieties of the mirabilis jalapa are crossed, one having red flowers and the other white, a fusion of the colours takes place in the first generation, and all the plants have pink flowers. In the second generation we get, for every plant with red flowers, two with pink flowers and one with white. That is, the law of disjunction has again asserted itself, but the individual hybrids merge their antagonistic attributes, which remain, nevertheless (as their differentiation proves), separate one from the other in the sexual cells.
Another phenomenon observed in individual hybrids is the intermingling of characteristics. For instance, there are cases where the flowers of a hybrid produced by a plant with red flowers and another with white are variegated with red and white stripes.
Accordingly, the transmission of antagonistic attributes through the individual may be divided into three different methods:
| Transmission | Exclusive. |
| By fusion. | |
| By intermingling. |
In the first case, the character of one of the parents is transmitted intact; in the second, the formation of a new characteristic results, constituting a form more or less nearly midway between those from which it comes and whose fusion it represents; in the third case (which is very rare and seems to obey Mendel's laws in quite an uncertain way), the result is a mosaic of the fundamental attributes.