Genetic factors: introduction

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 (stade 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.

Reminder

Gametogenesis

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
Karyotype	Phenotypical gender	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 and 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.