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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:
When two such gametes unite in fertilisation the resulting organism will be a homozygous recessive, and the corresponding character will be absent. In this way we can conceive the origin of albino individuals from a coloured race, supposing the colour was due to a single factor. In Bateson's opinion the origin of a new dominant is a much more difficult problem.
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,
In the horned Dorsets, we must suppose that both males and females carry a dual factor for horns technically, are homozygous for horns. The hornless Suffolks, on the contrary, are homozygous for absence of horns. If we represent horns by H and absence of horns by A, Dorsets have a factor HH, Suffolks AA and the hybrids HA.
The question remains, therefore, where are the factors of the somatic sex-characters? One suggestion which might be made is that the female characters are present in the Y, in this case female producing chromosome, or, if the female characters are merely negative, that the male characters are in the X chromosome, but only show themselves in the homozygous condition, thus:
Homozygous. B male B male X b female b male B male b female b male b male Barred female. Barred male. Heterozygous. This case is thus exactly similar to that of Abraxas grossulariata and A. lacticolor.
In Mendelian language the male is homozygous, so-called "pure" as regards this character. But the female is heterozygous, "impure" in the sense that her femaleness depends upon the dominance of the factor for femaleness over the factor for maleness, which also is present in her. In the Mendelian terminology, she is an instance of impure dominance.
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: When a normal male is mated with a heterozygous nN female we get that is, half the sons are normal and half colour-blind, while half the females are homozygous and normal, and the other half heterozygous and normal.
F1 Red-eyed male Red-eyed female XR XW XR YW F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female XR XR XW XR XR YW XW YW Homozygous. Heterozygous. Heterozygous. Homozygous. White-eyed male Red-eyed female XW XW x XR YW F1 Red-eyed male White-eyed female XW XR XW YW F2 White-eyed male Red-eyed male White-eyed female Red-eyed female XW XW XR XW XW YW XR YW Homozygous. Heterozygous.
Thus we are forced to conceive of the determinants in the chromosomes of the fertilised ovum which correspond to the two sides of the body, as entirely distinct from the determinants which cause the condition or 'characters' of the two sides, unless indeed we suppose that determinants of right side with eyes and colour occur in some gametes and of right side without eyes and colour in others, and vice versa, and that homozygous and heterozygous combinations occur in fertilisation.
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