Mendel's principles of heredity / by W. Bateson.

  • Bateson, William, 1861-1926.
Date:
1930
Catalogue details

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

Credit: Mendel's principles of heredity / by W. Bateson. Source: Wellcome Collection.

    179/470 (page 145)
    vili] Latency 145 been if the plant had been a coloured one. Lock (176) has spoken of this faint pattern as the ghost of the mapling. Mudge (204) has observed a very similar phenomenon in young albino rats. When the hair is short the coat may be seen to be similarly damasked, those parts which would be pigmented if the animal had pigment looking different in consistency from the rest. What the exact difference between the hairs in these areas and the rest may be has not been ascertained, but evidently it must be a modification due to the existence of one of the factors for colour in those hairs. They are the parts prepared to develop colour if the other element were present in them. In black leopards and black kittens a similar damask effect can often be seen, the parts which in the spotted leopard or tabby cat would be light being dis¬ tinguishable on careful examination. As it is not yet known whether black is dominant or recessive in these cases the exact meaning of these marks is uncertain. Both in animals and plants there is satisfactory proof that whiteness, the absence of colour, may be due to partial or complete suppression of the pigment-factors and not merely, as in the albino, to their absence. This suppression is caused by a dominant, epistatic factor. White individuals containing such a factor are more or less totally dominant whites, whereas whites due to the absence of one or more pigmentation-factors are recessive whites. A cross between a dominant and a recessive white may obviously cause coloured individuals to appear in 7%, if the factors for pig¬ mentation were introduced by the parental types ; for in there will be individuals lacking the suppressing factor. Confusion has been introduced into these analyses by the use of the term latency in application to those factors which cannot be perceived without breeding tests. This difficulty has occurred especially in regard to albinos, though it pervades the whole system of factorial analysis. Albinos, for instance, in any species may have the most diverse factorial composition. All that is common to them is the absence of colour, i.e., if we adopt Cuénot's suggestion, of the chromogenic substance. The composition of each albino may be ascertained by crossing it with a coloured type and raising the generation. If the coloured type chosen be B. H. 10