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Updated: June 6, 2025
When two gametes unite, the specific number is restored. Since the male gamete is very small and seems to contribute to the zygote almost nothing except the chromosomes, which carry with them all the characters of the male parent, it seems a necessary conclusion that the chromosomes alone determine the character of the adult. There are, however, facts which point to an opposite conclusion.
The majority of evolutionists in recent years have taught that influences exerted through the soma have no effect on the determinants in the chromosomes of the gametes, that all hereditary variations are gametogenic and none somatogenic.
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.
The meaning of a recessive character in Mendelian terminology is one that is hidden by a dominant character, and both of them are due to factors in the gametes, particularly in the chromosomes of the gametes which come together in fertilisation. For example, in fowls rose comb is dominant over single.
How comes it then that the female quality entirely disappears? Whether the gametocytes are distinguishable at an early stage in the segmentation of the ovum, or only at a later stage of development, we know that the gametes ultimately formed have descended by a series of cell-divisions from the fertilised ovum or zygote cell from which development commenced.
In Mendelian experiments, a heterozygote individual is one arising from gametes containing opposite members of a pair of characters, in other words, from the union of a gamete carrying a dominant with another carrying a recessive. A pure recessive individual is one arising from the union of two gametes both carrying recessives.
This theory, however, is still open to the objection that the female gametes before fertilisation, and half the male gametes, have the half quantity of sex-chromatin which by hypothesis determines the male condition, so that here again we have the male condition as something which is distinct from the characteristics of the spermatozoon. But if this is the case, what is the male condition?
If we represent the character of femaleness by F and maleness or the recessive by f, we have the ordinary sexual union represented by Ffxff; the gametes will then be F+f and f+f and the fertilisations Ff and ff, or males and females in equal numbers, as they are, at least approximately, in fact.
These results tend to show that factors are not indivisible units, and segregation is rather the difficulty of chromatin or germ plasm from different race uniting together. It must be remembered that the fertilised ovum which forms one individual gives rise also to dozens or hundreds or thousands or millions of gametes.
What we know in higher animals and plants is that each gamete contains in its nucleus half the number of chromosomes found in the other cells of the parent, and that in the fertilised ovum the chromosomes of both gametes form the new nucleus, in which therefore the original number of chromosomes is restored.
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