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.

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.

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.