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.

    60/392 (page 36)
    36 THE THEORY OF THE GENE present between the members of certain pairs—differ¬ ences in shape, or in the attachment to the spindle fibers (Fig. 23). When the germ-cells mature these chromo¬ somes conjugate and then separate. Since they retain their individual differences, they can be traced to the poles. Now in these grasshoppers there is, in the male, an un¬ paired chromosome that is connected with sex determina¬ tion (Fig. 23). It passes at the maturation division to one pole or to the other. It serves as a land-mark for the other pairs of chromosomes. Miss Carothers, who first made these observations, found that a marked pair (one bent, one straight) separates at random with respect to the sex chromosome. Carrying the matter further, other chromosome pairs were found to show at times constant differences in some individuals. A study of these chromosome pairs at reduc¬ tion has shown, again, a random distribution of the mem¬ bers of the pairs with respect to one another. Here then we have objective evidence of the independent assort¬ ment of the pairs of chromosomes. This evidence paral¬ lels Mendel's second law, which calls for independent dis¬ tribution of the members of different linkage groups. The Number of the Linkage Groups and the Number of the Chromosome Pairs. Genetics has shown that the hereditary elements are linked in groups, and in one case Avith certainty, and in several other cases with some probability, there is a defi¬ nite and fixed number of these linkage groups. In Droso- phila there are only four such groups, and there are four pairs of chromosomes. In the sweet pea there are seven chromosome pairs (Fig. 24), and probably seven inde¬ pendent pairs of Mendelian characters have been found by Punnett. In the edible pea there are also seven pairs