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Thus supposing M to represent a modification acquired in the male and m the absence of the modification, such as the feathered antenna of a moth, and the sex-chromosomes to be X and Y, then we should have in the gametocytes Male Female MM mm Gametes MX, MX: mXmY Zygotes MmXX male, MmXY female,
Or it might be somehow due to what Morgan and his colleagues have called crossing over in the segregation of heterozygous chromosomes, so that a part corresponding to a sinistral body is united with a part corresponding to a dextral head. My conclusion from the evidence is that any process of congenital development may in particular zygotes exhibit a mutation, a departure from the normal.
In the next generation m in the male would be affected, and the male for the sake of simplicity might be supposed to become MMXX. When the female gametes segregated, some would always be mY, and some zygotes therefore MXmY. Others might be MMXY. On this theory, therefore, there would always be some females heterozygous for the male character.
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.
Again, there are cases in which one sex-chromosome, say X, is double, triple, or even quadruple, while the other, say Y, is single. On fertilisation half the zygotes have XX and half XY, whether Y is absence of a sex-chromosome, or one of the other Y forms above mentioned.
In reuniting in fertilized eggs, the chance is just half and half that an H will unite with another H or with an A that an A will unite with an H or another A. Thus we have two chances of getting HA to each chance of getting either AA or HH. Half the zygotes will be HA, one-fourth HH and one-fourth AA.
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