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Exceptions may occur, however, and these are explained by what is called 'crossing over. When one chromosome of a pair, instead of being parallel to the other in the gametocyte, crosses it at a point of contact, then when the chromosomes separate, part of one chromosome remains connected with the part of the other on the same side and the two parts separate as a new chromosome, so that two factors originally in the same chromosome may thus come to lie in different chromosomes.
The gametes of the female each carry one X red chromosome, of those of the male half carry an X white chromosome, and half the Y white chromosome. The fertilised female ova therefore carry an X red chromosome + an X white chromosome, the male producing ova one X red chromosome and one Y white chromosome.
In that case the difficulty indicated in a previous part of this chapter, that the ovum after reduction resembles the sperm in having only one X chromosome, may be explained by the fact that the growth of the ovum and its accumulation of yolk substances has been already accomplished under the influence of the two chromosomes before reduction.
The important fact which is generally true for insects, according to Wilson, is that there is a special chromosome or chromosomes which can be distinguished from the others, and which is or are related to sex differentiation. This chromosome, to speak of it for convenience in the singular, has been variously named by different investigators.
The fertilisations are thus XX which develops into a female fly, and XY which develops into a male. Drosophila therefore is an example of one of the cases described by Wilson. Dr. He remarks that the X chromosome must be a male-determining factor since in many cases it is the only sex-chromosome in the males, yet its introduction into the egg establishes the female condition.
It would be unsafe to draw specific conclusions about mammals from these bird and insect experiments. Both the secretory action and the chromosome mechanisms are different. The quantitative nature of sex, and also the existence of intersexual types, between males and females, would seem to be general phenomena, requiring rather slight corroboration from the mammals themselves.
At the same time it must be borne in mind that since the factor, whether a portion of a chromosome or not, is transmitted in heredity as a part of a single cell, the gamete, and since every cell of the developed individual is derived by division from the single zygote cell formed by the union of the two gametes, the factor or determinant must be contained in every cell of the soma, except in cases where differential division, or what is called somatic segregation, takes place.
The original number of chromosomes in OEnothera is 14. In the mutation lata this has become 15, and also in another mutation called semilata. The chromosomes before the reduction division are arranged in pairs, each pair consisting, it is believed, of one paternal and one maternal chromosome.
Wilson points out that in the bee, where fertilised eggs develop into females and unfertilised into males, we should have to assume that the X chromosome in the female gamete is a female determiner which meets a recessive male determiner in the X chromosomes of the sperm. Dr.
When fertilization occurs, and the sperm cell fuses with the egg, the following may take place: a ten chromosome sperm may unite with the eleven chromosome egg, and produce a twenty-one chromosome individual or an eleven chromosome sperm may unite with an eleven chromosome egg producing a twenty-two chromosome individual.
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