Mendelism and evolution / by E.B. Ford.

  • Ford, E. B. (Edmund Briscoe), 1901-1988.
Date:
1931
Catalogue details

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

Credit: Mendelism and evolution / by E.B. Ford. Source: Wellcome Collection.

    33/144 page 17
    THE PARTICULATE THEORY 17 pelature affects the process) and with the constitution of the individual. In Drosophila, for example, certain genes prevent a breakage occurring in their immediate neighbourhood, while no crossing-over is possible in the males of this genus ; indeed in all forms it appears to be less frequent in the heterogametic than in the homo¬ gametic sex. On the other hand, the frequency with which a breakage and interchange may occur between any two factors in the same chromosome is constant in the same conditions. It is important very briefly to study the way in which crossing-over manifests itself, and the light which it throws on the mechanism of heredity. If two individuals, one homozygous for the dominant and the other for the recessive allelomorphs of two pairs of factors, are mated, double heterozygous offspring are produced. It has been pointed out that when these are back-crossed to the double recessive class, then, if the two pairs of factors are in different pairs of chromo¬ somes, free assortment takes place. The two grand- parental types and the two possible recombinations between them all appear in equal numbers. If, however, the factors are in the same pair of chromosomes, and complete linkage exists, the two grand-parental types only will segregate out. But when crossing-over occurs there appear in addition some individuals belonging to the recombination, or cross¬ over, classes. In order to estimate the amount of crossing-over, these are added together and expressed as a percentage of the whole. This is known as the ' cross-over value When the cross-over values of three factors situated in the same chromosome are studied, it is found that an important relationship exists between them. For the cross-over value between the factors A and В added
    digitised image 1
    digitised image 2
    digitised image 3