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Updated: May 12, 2025
On fertilisation two kinds of zygotes are formed, female-producing eggs with 2N chromosomes, and male-producing eggs with 2N-1 or 2N-2 chromosomes.
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:
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+.
If this explanation is correct, and it is in accord with careful biological observations, it removes from the mother all responsibility for the sex of her child. Furthermore, since the facts indicate that male-producing and female-producing spermatozoa are present in equal numbers, it follows that practically there is an even chance that an embryo will develop into a boy or a girl.
Overworking and other conditions tending to produce large eggs and females also throw white mutants and show other signs of weakness. Old females lay larger eggs than do young ones. These eggs produce more females. They store more material, have a lower metabolism and less oxidizing capacity than do the earlier male-producing eggs.
Wild pigeons begin the season by throwing a predominance of males, and the first eggs of the clutches also tend to produce males all along. In both cases, the male-producing eggs were found to be the ones with the smaller yolks. Family crosses also produce small yolks, which hatch out nearly all males. Some pairs of birds, however, have nearly all female offspring.
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