Genetics of Sex Determination

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B.K.A. Information pamphlet No.9 April 1966 C. Purdon.

The problem of what makes an animal (or a plant for that matter) one sex or the other is obviously very complex. In this article I want to consider chiefly the genetic aspects of the case, but some general comments are in order.

Sex is most important! This is obvious but what may be overlooked is that it is essential to be one sex or the other, intermediates are of little value. In addition, it is necessary that both sexes must be around at the same time, in order to reproduce their kind.

Various methods of sex-determination have been evolved by organisms in order to meet these advantageous requirements. Some methods are bizarre - thus in some deep sea fish the male is parasitic on the female and any young fish which makes contact with a female automatically develops into a male. In bees, and related insects, the fertilized egg with genetic material from both father and mother - develops into a female while the unfertilized egg ( maternal material only) develops into a male! The queen bee can lay whichever sort of egg she likes.

The genetic material which individuals inherit from their parents is carried on thread-like structures called chromosomes, these may be considered as books of coded information. Some species of animals and plants have very low numbers of chromosomes but fish and many other so-called higher organisms usually have twenty or more pairs of chromosomes. The coded information determines the way in which all aspects of the individual may develop and it is the interaction between these instructions and the many stimuli from the outside environment that determines the final form and function of an individual.

The extent to which environment determines the shape and function of an individual varies from one character to another. Environment obviously has a large effect on size or any measurable character but it has very little effect on other factors, for example, the number of fin rays. In the same way environment has very little effect on sex - the innate factors that determine sex are so strongly pro-male or pro-female that outside effects effects are normally negligible.

This is one way in which nature has evolved a system in which intermediate sexes are avoided.

To return to bees, it is the fact that the unfertilised egg carries only half the number of chromosomes that makes it develop into a male. This type of sex-determination is an extreme example of the commonest sort of mechanism, namely chromosomal sex-determination.

Almost all organisms show some form of chromosomal sex-determination.

In bees it involves an entire set of parental chromosomes but in most species it usually involves only one or often a small piece of one chromosome. This then leads to a very stable situation where the relative proportions of male and female remains equal and constant from one generation to another. Let us consider only that pair of chromosomes concerned with sex ( called sex - chromosomes as opposed to all the others called autosomes ) we may further classify them as X and Y chromosomes. Under this scheme one sex always had an identical pair of chromosomes (XX) the other has different chromosomes (XY). In most organisms including mammals and fish the male is XY, and the female XX. This condition is reversed in birds and in some butterflies.

Consider the inheritance of these chromosomes, A female (XX) passes one or the other chromosome (not both) into an egg which must therefore always contain one X chromosome. A male likewise passes one chromosome into each sperm cell but this time ( being XY ) it can be either X or Y. Thus the sperm cells are of two equally frequent types X or Y, and they fuse with one type of egg (X) to produce two types of fertilised egg XX or XY, thus the offspring are 50% XX and 50% XY or 50% female and 50% male. Very rarely one gets individuals in which things have gone wrong and which contains XXY or simply X.

These and other factors can upset the normal sex regulating machanisms, but this is another story and must await a further article.