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Updated: May 15, 2025
It is curious to note how different are these discoveries concerning differences in the number of chromosomes from the conception of Morgan that a mutation depends on a factor situated in a part of one chromosome. More copious details concerning mutations will be found in the publications cited.
Wilson called it the 'X chromosome, McCluny the 'accessory chromosome, Montgomery the 'hetero-chromosome, while the names 'heterotropic chromosome' and idiochromosome have also been used. For the purpose of the present discussion we may conveniently name it the sex-chromosome. It is often distinguished by its larger size and different shape. Wilson describes the following different cases:
All the factors of one group are contained in one chromosome, and it is found in experiments that the members of each group tend to be inherited together that is to say, if two or more enter a cross together, in other words, if a specimen possessing two or more mutations is crossed with another in which they are absent, they tend to segregate as though they were a single factor.
The X sex-chromosome is not in itself either female or male, since, as we have seen, either ovum or spermatozoon may contain a single X chromosome. The ovum then with one X chromosome or one X and one Y changes its sex at the next reduction division and becomes male.
Now when lata is produced it is believed that in the heterotypic division one pair passes into one daughter-cell instead of one chromosome of the pair into each daughter-cell, the other pairs segregating in the usual way. We thus have one daughter-cell with 8 chromosomes and the other with 6. This 6+8 distribution has actually been observed in the pollen mother-cell in rubrinervis.
They are all therefore red-eyed, but heterozygous that is, the red eye is due to one red-eye factor, not two. When the F1 are bred together, half the female gametes carry one X red chromosome, the other half one X white chromosome; half the male gametes carry one X red chromosome, the other half one Y white chromosome.
They demonstrate also the apparent chance nature of the phenomena of inheritance. To my mind the most striking and significant result in this field is the demonstration that a particular chromosome or chromatin mass determines a particular character of an adult organism, which is quite a different matter from the reference of all the hereditary characters to the chromatin as a whole.
Homozygous. Heterozygous. It must be explained that according to this theory the normal male is always heterozygous, because the Y chromosome never carries any other factor except that for sex; it is thus of no more importance than the absence of an X chromosome which occurs in those cases where the male has one sex-chromosome and the female two.
All the cells of the body, assuming that somatic segregation does not occur, must possess the same chromosomes as the zygote from which it developed, and whether the sex chromosomes are XX or XY or X, there must be at any rate one chromosome bearing the factor for milk glands.
To begin with, we must give some account of the difference between the cells of male and female origin, an unlikeness capable of producing the two distinct types of gametes, not only in external appearance, but in chromosome makeup as well.
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