Page:Mendel's principles of heredity; a defence.pdf/43

 in those two forms or varieties need not give any indication as to the character of the zygote Aa. It may display the character A, or a, or a character half way between the two, or a character beyond A or below a. The character of Aa is not regarded as a heritage transmitted to it by A and by a, but as a character special and peculiar to Aa, just as NaCl is not a body half way between sodium and chlorine, or such that its properties can be predicted from or easily stated in terms of theirs.

If a concrete case may help, a tall pea A crossed with a dwarf a often produces, not a plant having the height of either A or a, but something taller than the pure tall variety A.

But if the case obeys the Mendelian principles—as does that here quoted—then it can be declared first that the gametes of Aa will not be bearers of the character proper to Aa; but, generally speaking, each gamete will either bear the pure A character or the pure a character. There will in fact be a redistribution of the characters brought in by the gametes which united to form the zygote Aa, such that each gamete of Aa is pure, as the parental gametes were. Secondly this redistribution will occur in such a way that, of the gametes produced by such Aa's, on an average there will be equal numbers of A gametes and of a gametes.

Consequently if Aa's breed together, the new A gametes may meet each other in fertilisation, forming a zygote AA, namely, the pure A variety again; similarly two a gametes may meet and form aa, or the pure a variety again. But if an A gamete meets an a it will once more form Aa, with its special character. This Aa is the hybrid, or "mule" form, or as I have elsewhere called it, the heterozygote, as distinguished from AA or aa the homozygotes.