The theory of the gene / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1926
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The theory of the gene / by Thomas Hunt Morgan. Source: Wellcome Collection.

    34/392 (page 10)
    10 THE THEORY OF THE GENE are separated in the hybrid; for yellow and round that went in together have, in some cases, come out separated. Similarly for green and wrinkled. Mendel also showed that when three, and even four, pairs of characters enter a cross their elements are inde¬ pendently assorted in the germ-cells of the hybrid. It might, then, have seemed justifiable to extend this conclusion to as many pairs of characters as enter any particular cross. This would mean that there are as many independent pairs of elements in the germinal material as there are possible characters. Subsequent work has shown, however, that Mendel's second law of independent assortment has a more restricted application, since many pairs of elements do not assort freely, but certain ele¬ ments that enter together show a tendency to remain together in succeeding generations. This is called linkage. Linkage. Mendel's paper was recovered in 1900. Four years later Bateson and Punnett reported observations that did not give the numerical results expected for two inde¬ pendent pairs of characters. For instance, when a sweet pea having purple flower-color and long pollen grains is crossed to one with red flowers and round pollen grains, the two types that go in together come out together more frequently than expected for independent assortment of purple-red and round-long (Fig. 9). They spoke of these results as due to repulsion between the combinations purple and long and red and round, that came from oppo¬ site parents. Today these relations are called linkage. By linkage we mean that when certain characters enter a cross together, they tend to remain together in later generations, or, stated in a negative way, certain pairs of characters do not assort at random. It would seem, then, so far as linkage holds, that there