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He also discusses the case of Abraxas grossulariata and its variety lacticolor, and other cases of sex-linked heredity, apparently with the idea that all such cases are similar to those of sexual dimorphism.
This, of course, is contrary to the results of ordinary Mendelian experiments, for characters are inherited equally from individuals of either sex, except secondary sexual characters and sex-linked characters which we shall consider later.
One example in the barred character of the feathers in the breed of fowls called Plymouth Rock. In this the female is heterozygous for sex as in Abraxas grossulariata, and the barred character is sex-linked. When a barred hen is crossed with an unbarred cock all the male offspring are barred, all the females plain.
The factor for red eye occurs in the sex-chromosomes, that is to say, according to this theory, the sex-chromosome does not merely determine sex but carries other factors as well, and this fact is the explanation of sex-linked inheritance. The factor for red eye then is present in both X chromosomes of the wild female, absent from both X and Y chromosomes of the white-eyed male.
That characters do originate by the first of these processes may be considered to be proved by recent researches, and such characters are called mutations. There can be little doubt that the so- called sex-linked characters, of which examples have been given above, have originated in this way, and that their relation to sex is part of the mutation.
The evidence concerning sex and sex-linked characters and the localisation of their factors in the chromosomes of the gametes has no bearing on the action of hormones. The facts concerning the action of hormones are beyond the scope of current conceptions of the action of factors or genes localised in the gametes and particularly in the chromosomes.
It is obvious from the above facts that however interesting and important sex-linked heredity may be, it is not the same thing as the heredity of secondary sexual characters, and does not in the least explain sexual dimorphism.
In the first place, the term sex-linked does not mean occurring always exclusively in one sex, but the direct contrary transmitted by one sex to the opposite sex and in the second place there is no suggestion that the development of the character is dependent in any way on the presence or function of the gonad.
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