21.1 Genetic factors and hormones that determine gender

Genetic factors: introduction


Quiz 01

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.



Comparison table: aneuploidy and euploidy of the gonosomes
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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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
Turner's syndrome

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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Special case of a dissociation between the karyotype and phenotype.

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