SEX DETERMINATION

Males have 46 chromosomes with an X and a Y forming the pair of sex chromosomes. The female also has the same number of 46 chromosomes with two X chromosomes forming the pair of sex chromosomes. There will then be two types of spermatozoa, those carrying an X chromosome included in the halved number of 23 and those carrying a Y chromosome. All the female ova however will be of one type carrying 23 chromosomes which includes an X chromosome. Depending on whether an X spermatozoon or a Y spermatozoon fertilizes any of the X ova, an XX female zygote or an XY male zygote will be formed. The male therefore is responsible for the sex of the zygote and the chromosomal sex and the genetic sex are determined at fertilization.

During the development of the zygote, before the seventh week, the developing gonads in both sexes are identical in appearance and are undifferentiated. In an XY zygote the Y chromosome has a gene in its short arm referred to as a testis-determining factor (TDF) located in a region referred to as the "sex determining region of the Y" (SRY). Under the influence of this gene, the primary sex cords in the medulla of the undifferentiated gonad differentiate into seminiferous tubules. In an XX zygote the mere absence of a Y chromosome is sufficient for the differentiation of the gonad into an ovary. However, complete development of the ovary requires the presence of both X chromosomes.

The gonadal sex thus established, determines the type of sexual differentiation that takes place in the genital ducts and the external genitalia. This establishes the genital sex. The androgen testosterone produced by the foetal testis determines maleness. The primary female sexual differentiation in the foetus however does not depend on hormones but does later on oestrogens. The physical sex and the social sex are thus established