Evolution and Adaptation

by Thomas Hunt Morgan

The importance of Mendel’s results and their wide application is apparent from the results in recent years of De Vries, Correns, Tschermak, Bateson, Castle, and others. Mendel carried out his experiments on the pea, Pisum sativum. Twenty-two varieties were used, which had been proven by experiment to be pure breeds. When crossed they gave perfectly fertile offspring. Whether they all have the value of varieties of a single species, or are different subspecies, or even independent species, is of little consequence so far as Mendel’s experiments are concerned. The flower of the pea is especially suitable for experiments of this kind. It cannot be accidentally fertilized by foreign pollen, because the reproductive organs are inclosed in the keel of the flower, and, as a rule, the anthers burst and cover the stigma of the same flower with its own pollen before the flower opens. In order to cross-fertilize the plants it is necessary to open the young buds before the anthers are mature and carefully remove all the anthers. Foreign pollen may be then, or later, introduced.

The principle involved in Mendel’s law may be first stated in a theoretical case, from which a certain complication that appears in the actual results may be removed.

If A represent a variety having a certain character, and B another variety in which the same character is different, let us say in color, and if these two individuals, one of each kind, are crossed, the hybrid may be represented by H. If a number of these hybrids are bred together, their descendants will be of three kinds; some will be like the grandparent, A, in regard to the special character that we are following, some will be like the other grandparent, B, and others will be like the hybrid parent, H. Moreover, there will be twice as many with the character H, as with A, or with B.

If now we proceed to let these A’s breed together, it will be found that their descendants are all A, forever. If the B’s are bred together they produce only B’s. But when the H’s are bred together they give rise to H’s, A’s, and B’s, as shown in the accompanying diagram. In each generation, the A’s will also breed true, the B’s true, but the H’s will give rise to the three kinds again, and always in the same proportion.

Thus it is seen that the hybrid individuals continue to give off the pure original forms, in regard to the special character under consideration. The numerical relation between the numbers is also a striking fact. Its explanation is, however, quite simple, and will be given later.

In the actual experiment the results appear somewhat more complicated because the hybrid cannot be distinguished from one of the original parents, but the results really conform exactly to the imaginary case given above. The accompanying diagram will make clearer the account that follows.

The hybrid, A(B), produced by crossing A and B is like A so far as the special character that we will consider is concerned. In reality the character that A stands for is only dominant, that is, it has been inherited discontinuously, while the other character, represented by B, is latent, or recessive as Mendel calls it. Therefore, in the table, it is included in parentheses. If the hybrids, represented by this form A(B), are bred together, there are produced two kinds of individuals, A’s and B’s, of which there are three times as many A’s as B’s. It has been found, however, that some of these A’s are pure forms, as indicated by the A on the left in our table, while the others, as shown by their subsequent history, are hybrids, A(B). There are also twice as many of these A(B)’s as of the pure A’s (or of the B’s). Thus the results are really the same as in our imaginary case, only obscured by the fact that the A’s and the A(B)’s are exactly alike to us in respect to the character chosen. We see also why there appear to be three times as many A’s as B’s. In reality the results are 1 A, 2 A(B), 1 B.

In subsequent generations the results are the same as in this one, the A’s giving rise only to A, the B’s to B, and the A(B)’s continuing to split up into the three forms, as shown in our diagram. Mendel found the same law to hold for all the characters he examined, including such different ones as the form of the seed, color of seed-albumen, coloring of seed-coat, form of the ripe pods, position of flowers, and length of stem.