Note 3 — Chapter V

In addition to these appeared Himalayans with either black points or with lighter brownish ones, and the proportions in which they came showed the Himalayan character to be a simple recessive. A certain number of the coloured forms exhibited the Dutch marking to a greater or less extent, but as its inheritance in this set of experiments is complicated and has not yet been worked out, we may for the present neglect it and confine our attention to the coloured types and to the Himalayans. The proportion in which the four coloured types appeared in F2 was very nearly 9 agoutis, 3 blacks, 3 yellows, and 1 tortoiseshell. Evidently we are here dealing with two factors: (1) the grey factor (G), which modifies black into agouti, or tortoiseshell into yellow; and (2) an intensifying factor (I), which intensifies yellow into agouti and tortoiseshell into black. It may be mentioned here that other experiments confirmed the view that the yellow rabbit is a dilute agouti, and the tortoiseshell a dilute black. The Himalayan pattern behaves as a recessive to self-colour. It is a self-coloured black rabbit lacking a factor that allows the colour to develop except in the points. That factor we may denote by X, and as far as it is concerned the Himalayan is constitutionally xx. The Himalayan contains the intensifying factor, for such pigment as it possesses in the points is full coloured. At the same time it is black, i.e. lacking in the factor G. With regard to these three factors, therefore, the constitution of the Himalayan is ggIIxx. The last character which we have to consider in this cross is the Dutch character. This was found by Hurst to behave as a recessive to self-colour (S), and for our present purpose we will regard it as differing from a self-coloured rabbit in the lack of this factor.[3] The Himalayan is really a self-coloured animal, which, however, is unable to show itself as a full black owing to its not possessing the factor X. The results of breeding experiments then suggest that we may denote the Himalayan by the formula ggIIxxSS and the yellow Dutch by GGiiXXss. Each lacks two of the factors upon the full complement of which the agouti colour depends. By crossing them the complete series GIXS is brought into the same zygote, and the result is a reversion to the colour of the wild rabbit.