Page:Popular Science Monthly Volume 84.djvu/10

6 characters are involved. If one of the original parents had round seeds and the other wrinkled seeds, these characters separate independently of the green-yellow separation. We say technically that the pair yellow and green, and the pair round and wrinkled segregate independently of each other. Chance combinations of the germ-cells in a double hybrid of this sort give nine yellow round peas, three yellow wrinkled peas, three green round peas and one green wrinkled pea (Fig. 2).

The same rule applies to three or more pairs of characters. Mendel assumed in fact that independent assortment always takes place no matter how many characters are involved.

In more recent times evidence has been accumulating which shows that the chromosomes are the bearers of most of the elements (factors) that produce those characters that we study in heredity. I can not take up the work that seems to me to place this hypothesis on a very probable basis, but I shall simply assume that it is a reasonable conclusion from the evidence at hand.

 Fig. 1. Diagram to illustrate Mendel's law of segregation. Individuals (zygotes) are represented by superimposed circles, whose colors stand for the factors involved. Gametes (germ-cells) are represented by single circles.