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 5 for it is also found in one quarter of the F2 generation. The remainder, half in all, are heterozygotes similar to the Fl individuals. Consequently they do not breed true but, as before, when interbred one quarter of their offspring have red eyes. This result is at once intelligible on the chromosome hypothesis. The true breeding black-eyed and red- eyed conditions are each dependent on a pair of alíelo- morphic factors homozygous for the genes concerned. If the factor for black eyes be represented by R and that for red eyes by r, then each cell of the pure black- eyed animal must contain the factors RR, and of the red-eyed rr. These pairs, being allelomorphic, are situated in homologous chromosomes, and they separ¬ ate from each other at the maturation division when the chromosome number is halved. In consequence, the germ cells of the pure black- and red-eyed types contain only one member of each factor-pair, R and r respectively. When a black-eyed and a red-eyed Gammarus are mated, the original number of chromosomes, which has been halved in the germ cells, is of course restored at fertilization. The character with which we are con¬ cerned, facet colour, is now once more represented by a fair of factors. But this is a heterozygous pair of the constitution Rr, made up of R from one parent and r from the other. At maturation, therefore, when the allelomorphs separate again, the germ cells of this, the n, generation receive either the factors R or r. The chances therefore are equal that either type of ovum may be fertilized by either type of spermatozoon. That is to say, a spermatozoon carrying R may meet an ovum carrying R or r, so giving either the combination RR or Rr ; so also may one of the equally numerous f-bearing spermatozoa, giving either the combination 2
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