Mendelism / by R.C. Punnett.

  • Punnett, Reginald Crundall, 1875-1967.
Date:
1912
Catalogue details

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

Credit: Mendelism / by R.C. Punnett. Source: Wellcome Collection.

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    IV PRESENCE & ABSENCE THEORY 29 than one of the two factors belonging to the alter¬ native pair. On the other hand, Mendel supposed that it always carried either one or the other of such a pair. As experimental work proceeded, it soon became clear that there were cases which could not be expressed in terms of this conception. The nature of the difficulty and the way in which it was met will perhaps be best understood by considering a set of experiments in which it occurred. Many of the different breeds of poultry are characterised by a particular form of comb, and in certain cases the inheritance of these has been carefully worked out. It was shown that the rose comb (Fig. 4, B) with its flattened papillated upper surface and back- wardly projecting pike was dominant in the ordinary way to the deeply serrated high single comb (F iS- 4> C) which is characteristic of the Mediterranean races. Experiment also showed that the pea comb (Fig. 4, A), a form with a low central and two well-developed lateral ridges such as is found in Indian game, behaves as a simple dominant to the single comb. The inter¬ esting question arose as to what would happen when the rose and the pea, two forms each dominant to the same third form, were mated together. It seemed reasonable to suppose that things which were alter¬ native to the same thing would be alternative to one another—that either rose or pea would dominate in the hybrids, and that the F^ generation would consist of dominants and récessives in the ratio 3:1. The result of the experiment was, however, very different. The cross rose x pea led to the production of a comb quite unlike either of them. This, the so-called walnut comb (Fig. 4, D), from its resemblance to