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Updated: May 15, 2025
This has included sex phenomena since McClung and Sutton pointed out the function of the sex chromosome in 1902 and 1903. Present-day "theories" are little more than working hypotheses, developed, not in a library or study, but with one eye glued to a high-power microscope.
The recessive character in this case is linked to the female sex chromosome, or, as Bateson described it, the dominant character is repelled by the sex-factor. We may make a diagram of the kind given by Morgan if we use a rod of different shape for the female-producing sex-chromosome, and use the black rod for the dominant character:
Should the division occur across the thread the two halves would be unlike, but taking place as it does by a longitudinal splitting each unit in the thread simply divides in half, and thus the resulting half threads each contain the same number of similar units as the other and the same as possessed by the original undivided chromosome.
We can only suppose that the final development of the sperms is the result of the presence of the single X chromosome in the successive generations of male gametocytes before the reduction divisions.
The zygote therefore, whether the sex of it is determined as male or female, has the same factor for the development of milk glands. On the chromosome theory as formulated by Morgan this factor must be in the somatic chromosomes and not in the sex-chromosomes, and must be present in every zygote.
On the other hand, we have the evident fact that a number of chromosomes formed apparently of the same substance, by a series of equal chromosome divisions determine all the various special parts of the complicated body.
Various attempts have been made to explain gynandromorphism in insects in accordance with the chromosome theory of sex-determination. These are discussed by Doncaster in the volume already cited, but from the point of view of the present work the important question is that concerning the somatic sex-characters.
This is the process of maturation. In the process, when the chromosome number is halved among the females, 11 go into each mature egg. But among the males, the odd chromosome, also known as the X-chromosome, can perforce go only into half of the sperm cells, leaving the others without it. So the sperm are formed in equal numbers of 10 and 11 chromosomes respectively.
One of each pair goes into one daughter-cell and the other into the other, but not all maternal into one and all paternal into the other. Thus each daughter-cell after the first or heterotypic division in normal cases contains 7 chromosomes. A second homotypic division takes place in which each chromosome splits into two as in somatic divisions, and thus we have 4 gametes with 7 chromosomes each.
FEMALE x MALE XY XX | \/ | | /\ | XX YX MALE FEMALE The male characters in the male, XX, would appear because present in two chromosomes, but would be recessive in the female because present only in one chromosome.
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