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.

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    THE PARTICULATE THEORY 7 ance is imperfect or complete, is characteristic of the F2 generation. The facts so far described can now be summarized in two diagrams (Figs. 1 and 2). B. Independent Assortment. In his second law Mendel stated that two contrasted characters behave independently of each other when inherited together. From what has so far been said of the mechanism of heredity the truth of this statement must be obvious, so long as the factor-pairs in question are situated in different pairs of chromosomes. In such circumstances their allelomorphs will segregate from each other at the reduction division and pass into the germ cells at random. It is evident that when individuals with two con¬ trasted pairs of characters, each exhibiting complete dominance, are crossed, four types will be produced in the F2 generation. These represent the original distri¬ bution of the characters and the two possible recom¬ binations between them. Since each type, considered separately, will appear in a 3 : 1 ratio in this generation, the four will together be present in a combination of two such ratios ; that is to say in a ratio of 9 : 3 : 3 : 1. To make this plain, we will consider the example of the pair already studied, black (dominant) compared with red facets in the eyes of Gammarus chevreuxi, and combine it with another pair of factors of quite similar inheritance carried in different chromosomes. Such are the factors controlling the appearance of white pigment surrounding the facets ; the presence of this pigment, the normal condition, being dominant, and its absence recessive. If a Gammarus pure-bred for the black-with-white characters be mated with one having red facets and no surrounding white pigment, the offspring will receive one member of each of the chromosome pairs carrying