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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.
There is also evidence that in some cases, e.g. the sea-urchin, the female is heterozygous, forming gametes, some with N and some with N+ chromosomes, while the male gametes are all N. Fertilisation then produces male-producing eggs with 2N chromosomes, female-producing with 2N+.
This is the same difficulty which I pointed out above in connection with the Mendelian theory that the female was heterozygous and the male homozygous for sex. Dr.
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:
On the other hand, if a barred cock is crossed with an unbarred hen, the barred character appears in all the offspring, both and females. The female thus transmits the character only to her sons. If we represent the barred character by B, and its absence by b, we can represent the heredity as follows: B female b male X b male b male Bb male bb female Barred male. Unbarred female. Heterozygous.
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