3. Meiotic Drift

It is well established in cyto-genetics, that there is an active process which leads to genetic drift within a population. This happens during meiosis as a result of crossing-over, when during the exchange of chromosomes, factors distribute themselves in ratios different from those postulated by Mendel's laws. Experiments on fruit-flies have shown how meiotic drift can be induced (for example, by spirochaeta infection, and by certain mutagens), and to what extent it can lead to a change in the ratio of genders (in some cases, the percentage of female descendents along the male line can be as high as 95%).

Crossing-over occurs at those phases of meiosis where cell-division is declining - in the stage where spermatocyte-1 transforms into spermatocyte-2 (diagram 2). It is well known that these very spermatocytes are the target cells for testosterone. Crossing-over, then, is a testosterone-dependant process.

The level of testosterone production by Glandulocytes (Leidig cells), which have direct functional links to cells of the sperm-generating epithelium, is determined by genetic factors and by the intensity of sexual life.

Diagram 2: The Cytogenetic Mechanism of pre-zygotic sex determination

4. The Cytogenetic Basis of pre-zygotic sex determination

In accordance with the above, the cyto-genetic basis of pre-zygotic gender determination can be seen as a testosterone dependant process - crossing-over during the course of meiosis. During this process, the distribution and exchange of sex-chromosomes occurs according to Mendel's laws, but the determination of the functional characteristics of X- and Y-carrying spermatozoids does not. As a result of meiotic drift, the surface membranes of X- and Y-carrying sperm acquire special characteristics which are realized during immunological reactions after the sperm enter the woman's genital tract.

In the male organism, then, there is a biological 'tool,' the purpose of which is to determine the functional characteristics of X- and Y- carrying sperm cells so that one pool of sperm gains an advantage over others during the movement towards the egg-cell, and another pool of sperm does not gain such an advantage. Clearly, this is a universal mammalian biological mechanism which is regulated by hormones, and can be the subject of external manipulation.

Note that the above-defined cyto-genetic phenomenon occurs in the male organism. In this connection, we should turn our attention to the sexual dimorphism of spermatogenesis and ovogenesis.