Scheme of inheritance in the F1 and F2 generations resulting from the cross of lacticolor female with grossulariata male. The character of each individual is represented by the sex signs in brackets, the black being grossulariata in appearance and the light ones lacticolor.
We may now construct a scheme for comparison with that on page [100] to show how these assumptions explain the experimental results. The original parents were lacticolor female and grossulariata male, which on our assumptions must be Ffgg and ffGG respectively in constitution. Since the female is always heterozygous for F, her gametes must be of two kinds, viz. Fg and fg, while those of the pure grossulariata male must be all fG. When an ovum Fg is fertilised by a spermatozoon fG, the resulting zygote, FfGg, is heterozygous for both F and G, and in appearance is a female grossulariata. The zygote resulting from the fertilisation of an ovum fg by a spermatozoon fG is heterozygous for G, but does not contain F, and therefore is a male grossulariata. Such a male being in constitution
ffGg must produce gametes of two kinds, fG and fg, in equal numbers. And since we are assuming repulsion between F and G, the F1 female being in constitution FfGg, must produce equal numbers of gametes Fg and fG. For on our assumption F and G cannot enter into the same gamete. The series of gametes produced by the F1 moths, therefore, are fG, fg by the male and Fg, fG by the female. The resulting F2 generation consequently consists of the four classes of zygotes Ffgg, FfGg, ffGg, and ffGG in equal numbers. In other words, the sexes are produced in equal numbers, the proportion of normal grossulariata to lacticolor is 3 : 1, and all of the lacticolor are females; that is to say, the results worked out on our assumptions accord with those actually produced by experiment. We may now turn to the results which should be obtained by crossing the F1 moths with the lacticolor variety. And first we will take the cross lacticolor female × F1 male. The gametes produced by the lacticolor female we have already seen to be Fg and fg, while those produced by the F1 male are fG and fg. The bringing together of these two series of gametes must result in equal numbers of the four kinds of zygotes FfGg, Ffgg, ffGg, and ffgg, i.e. of female grossulariata and lacticolor, and of male grossulariata and lacticolor in equal numbers. Here, again, the calculated results accord with those of experiment. Lastly, we may examine what should happen when the F1 female is crossed with the lacticolor
male. The F1 female, owing to the repulsion between F and G, produces only the two kinds of ova Fg and fG, and produces them in equal numbers. Since the lacticolor male can contain neither F nor G, all of its spermatozoa must be fg. The results of such a cross, therefore, should be to produce equal numbers of the two kinds of zygote Ffgg and ffGg, i.e. of lacticolor females and of grossulariata males. And this, as we have already seen, is the actual result of such a cross.
Before leaving the currant moth we may allude to an interesting discovery which arose out of these experiments. The lacticolor variety in Great Britain is a southern form and is not known to occur in Scotland. Matings were made between wild Scotch females and lacticolor males. The families resulting from such matings were precisely the same as those from lacticolor males and F1 females, viz. grossulariata males and lacticolor females only. We are, therefore, forced to regard the constitution of the wild grossulariata female as identical with that of the F1 female, i.e. as heterozygous for the grossulariata factor as well as for the factor for femaleness. Though from a region where lacticolor is unknown, the "pure" wild grossulariata female is nevertheless a permanent mongrel, but it can never reveal its true colours unless it is mated with a male which is either heterozygous for G or pure lacticolor. And as all the wild northern males are
pure for the grossulariata character this can never happen in a state of nature.
Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn cock. Black sex signs denote deeply pigmented birds, and light sex signs those without pigmentation. The light signs with a black dot in the centre denote birds with a small amount of pigment.
An essential feature of the case of the currant moth lies in the different results given by reciprocal crosses. Lacticolor female × grossulariata male gives grossulariata alone of both sexes. But grossulariata female × lacticolor male gives only grossulariata males and lacticolor females. Such a difference between reciprocal crosses has also been found in other animals, and the experimental results, though sometimes more complicated, are explicable on the same lines. An interesting case in which three factors are concerned has been recently worked out in poultry. The Silky breed of fowls is characterised among other peculiarities by a remarkable abundance of melanic pigment. The skin is dull black, while the comb and wattles are of a deep purple colour contrasting sharply with the white plumage (Pl. V., 3). Dissection shows that this black pigment is widely spread throughout the body, being especially marked in such membranes as the mesenteries, the periosteum, and the pia mater surrounding the brain. It also occurs in the connective tissues among the muscles. In the Brown Leghorn, on the other hand, this pigment is not found. Reciprocal crosses between these two breeds gave a remarkable difference in result. A cross between the Silky hen and the Brown Leghorn cock produced F1 birds in which both sexes exhibited only traces of the pigment. On casual observation they might have
passed for unpigmented birds, for with the exception of an occasional fleck of pigment their skin, comb and wattles were as clear as in the Brown Leghorn (Pl. V., 1 and 4). Dissection revealed the presence of a slight amount of internal pigment. Such birds bred together gave some offspring with the full pigmentation of the Silky, some without any pigment, and others showing different degrees of pigment. None of the F2 male birds, however, showed the full deep pigmentation of the Silky.