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21.1 Genetic factors and hormones that determine gender
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Genetic factors: introduction
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In the nucleus of every cell of his or her body, a human being has 46 chromosomes. 22 chromosome pairs (numbered from 1-22) belong to the autosomes and 1 pair to the sex chromosomes or gonosomes. They are denoted as X and Y. A female has two X-chromosomes and a male an X and a Y-chromosome. In a woman, one of the two X-chromosomes is inactivated in the form of heterochromatin (sex chromatin), the Barr body - diagnosis of the genetic gender is made on this basis. This inactivation already takes place in the blastocyst stage 3 - randomly - either on the paternal or maternal X chromosome. When a Y chromosome is present, the development takes place in the direction of manhood; if it is missing, a feminine development occurs.
It is not the number of gonosomes that is decisive for the gender, but rather the presence or absence of the Y-chromosome, as can be seen in the following table.
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Comparison table: aneuploidy and euploidy of the gonosomes |
Karyotype |
Phenotypical
gender
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Gonad |
Syndromes |
Fate |
45, XO |
female |
Ovaries |
Turner's syndrome |
Atrophy of the ovaries in the fetus |
45, YO |
--- |
--- |
--- |
Absence of the X -chromosome is lethal |
46, XX |
female |
Ovaries |
Normal woman |
Normal development |
47, XXX |
female |
Ovaries |
Normal fertility |
Normal development |
46, XY |
male |
Testes |
Normal man |
Normal development |
47, XXY |
male |
Testes |
Klinefelter's syndrome |
Small testes, aspermatogenesis |
47, XYY |
male |
Testes |
Normal fertility |
Normal development |
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More info
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An aneuploidy (anomaly in the number of chromosomes) of the gonosomes (sex chromosomes) is not rare, whereby Klinefelter's syndrome and Turner'ssyndromes occur the most frequently.
Klinefelter's syndrome
and
Turner's syndrome
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It is clear that the information encoded on the Y-chromosome is not enough to guide the formation of such a complicated organ as the testicles, but a localized gene on this chromosome, the SRY (sex determining region Y gene) operates very early in the development as a guide or "master gene". It has a testis-determining effect on the indifferent gonads. This small gene (a single exon), which is localized on the shorter arm of the Y chromosome (Yp), gets expressed in the precursors for the supporting cells (Sertoli). It controls a whole number of further genes on the autosomes as well as on the X chromosome. It is only through the concerted workings of this SRY-gene together with genes on other chromosomes that the development of the testicles is possible. (Diagram of the molecular factors involved in the development of the genital apparatus)
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More info
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Special case of a dissociation between the karyotype and phenotype.
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