2. Pre-zygotic selection

Immunological factors doubtlessly have a great effect on conception. We have evidence of this fact from the investigation of rabbits, immunized with spermatozoid antigens. The effect of the antigens drastically altered the gender-ratio among descendents along the female line. When mating took place after the introduction of X-sperm antigens, more males were born, and when Y-sperm antigens were introduced, more females.

In the immunology of reproduction it is well established that significant differences exist in the characteristics of sperm carrying the different chromosomes (references 12,13,14). The protein on the surface of male sex-cells has different characteristics in X- and Y-carrying sperm. The particular characteristics of this membrane are genetically controlled, as indeed are all the other characteristics of spermatozoids. On the other hand, some factors, such as sperm's speed of motion are dependant on the specifics of the surrounding environment. On the basis of the functional and morphological differences between X- and Y-carrying sperm cells, some methods have been proposed for distinguishing between the two, with the goal of determining the sex of a baby by artificial insemination.

The movement of sperm along the sexual channels before and after ejaculation are accompanied by complex immunological phenomena which are known as 'pre-zygotic selection'.

Pre-zygotic selection is seen as the combination of two processes which occur in the woman's sexual tract after ejaculation. Firstly (while sperm move through the cervical filter), genetically defective sperm are eliminated by immunological reactions. After this 'rejection of the defective,' there follows 'selection of the fittest.' The essential feature of this last process in that from the pool of genetically viable sperm, only the 'best' (according to certain functional characteristics), are allowed to move towards the egg for fertilization.

Diagram 1: Pre-zygotic selection: 1 - "rejection of the defective"; 2 - "selection of the fittest"
(by N.N. Zhukov-Verezhnikov, et al.,1979)

It is by no means impossible that the chromosome carried by a sperm-cell (i.e. X or Y), could have an effect on its selection or rejection. The theoretical positions of reproductive immunology, however, do not explain how exactly X- or Y- carrying sperm could acquire such a characteristic. To find the explanation we must turn to ideas about the regulation of spermatogenesis, and especially to the genetic phenomenon of meiotic drift.