Bateson proceeds to consider colour-blindness as though its heredity were of similar kind. He refers to it as a male character latent in the female, remarks that we should expect that disease or removal of the ovaries might lead to the occasional appearance of colour-blindness in females. 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. A. lacticolor occurs in nature only in the female sex, and when bred with grossulariata [male] produces [male]'s and [female]'s all grossulariata, these of course being heterozygous. When the F1 grossulariata [male] was bred with the wild lacticolor [female] it produced both forms in both sexes, and thus lacticolor [male] was obtained for the first time. When this lacticolor [male] was bred with F1 grossulariata[female] it produced all the [male]'s grossulariata and all the [female]'s lacticolor. Bateson's explanation is that the female, according to the Mendelian theory of sex, is heterozygous in sex, the male homozygous and recessive, and that lacticolor is linked with the female sex-character, grossulariata being repelled by that character. Thus we have, the lacticolor character being recessive,
lact. male, LL male male x F, gross. female, GL female male
Gametes L male + L male x G male + L female
_____________________|______________________
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GL male male LL male female
gross. male lact. female
It will be seen that although in the progeny of this mating all the grossulariata were males and all the lacticolor females, yet this case is by no means similar to that of sexual dimorphism in which the characters are normally always confined to the same sex. For the lacticolor character in the parent was in the male, while in the offspring it was in the female. We cannot say here that in the theoretical factors which are supposed to represent what happens, the lacticolor character is coupled with the female sex-factor, for we find it with the male sex-character in the lacticolor [male]. It is so coupled only in the heterozygous grossulariata [female], and at the same time the grossulariata character is repelled.
According to Doncaster [Footnote: Determination of Sex, Camb. Univ. Press, 1914.] sex-limited, or as it is now proposed to call it sex-linked, transmission in this case means that the female grossulariata transmits the character to all her male offspring and to none of the female, while a heterozygous male grossulariata mated with lacticolor female transmits the character equally to both sexes: that is to say, the heredity is completely sex-limited in the female but not at all in the male. This is evidence that the female produces two kinds of eggs, one male producing and the other female producing.
With regard to the ordinary form of colour-blindness, Bateson's first explanation was that it was like the horns in the cross-bred sheep, dominant in males, recessive in females. About 4 per cent. of males in European countries are colour-blind, but less than 1/2 per cent. of females. Affected males may transmit the defect to their sons but not to their daughters: but daughters of affected persons transmit the defect frequently to their sons. Bateson gives [Footnote: Mendel's Principles of Heredity, 1909.] a scheme of the transmission, but corrects this in a note stating that colour-blindness does not descend from father to son, unless the defect was introduced by the normal sighted mother also, i.e. was carried by her as a recessive. The fact that unaffected males do not transmit the defect shows, according to Bateson, that it is due to the addition of a factor to the normal, not to omission of a factor.
According to later researches as quoted by Doncaster, colour-blindness is due to the loss of some factor which is present in the normal individual. The normal male is heterozygous for this normal factor. If we denote the presence of the normal factor by N and its absence or recessive by n, then the male is Nn, while the female is homozygous or NN. But in addition to this it is the male in this case which is heterozygous for sex, and n goes to the male-producing sperms, N to the female-producing. Thus in the mating of normal man with normal woman the transmission is as follows:—
Nn (male) x NN (female)
Gametes n (male) + N (female) x N + N
n (male) + N N (female) + N
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Nn (male) NN (female)
That is all offspring normal, but the males again heterozygous.
An affected male has the constitution nn, and if he marries a normal woman the descent is as follows:—