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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    16 MENDELISM AND EVOLUTION random among the sexes as the characters previously considered have been. Half the males, but none of the females, have white eyes like the male grandparent. The appearance of the white-eye character is thus linked with the male sex though inherited through the female. It should be noticed, however, that the appearance of sex-linked characters is not confined to one sex. They appear more frequently in the heterogametic than in the homogametic sex, because in general the effect of récessives cannot be swamped by a dominant partner in the Y-chromosome. In the present instance white- eyed females could easily be produced, by mating females heterozygous for this character to white-eyed males. Fig. 6 also demonstrates the important fact, pre¬ viously noticed, that sex is determined by a quanti¬ tative reaction depending merely on the number of X-chromosomes present. For it is seen that the single X-chromosome of the male is derived from his mother, where it was concerned in producing the female con¬ dition. That the Y-chromosome does not determine sex in Drosophila is demonstrated by exceptional instances in which it is lost ; this does not affect the development of the male. E. Crossing - over. Linkage, however, is not always complete, for an exchange of chromatin, and therefore of genes, may occur between the homologous chromosomes when they conjugate with each other pre¬ paratory to the reduction division. This is known as Crossing-over. At this time the chromosomes become spun out into thin threads (the leptotene stage) which twist round each other ; and the evidence indicates that they break and exchange blocks of material at one or more of the points of contact. The frequency with which this may occur varies with the environment (tem-
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