BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.8. Human Sexual Reproduction

21.8.11. Sex Determination in the Developing Embryo

The sex of an embryo is determined by the sex Chromosomes carried by the sperm. The X chromosome determines female sex, and the Y chromosome determines male sex. All egg Cells contain a single X chromosome, whereas sperm cells contain either an X or a Y chromosome. Consequently, the zygote formed upon gamete fusion has either an XX (female) or an XY (male) genotype (see Section 24.6). In the early Selection/3.html">Stages of development, the embryo develops a pair of undifferentiated Gonads known as genital ridges. Up to the sixth week, no structural differences are observed between male and female gonads, but thereafter the sex chromosomes come into play, determining The pathway of Reproductive System development.

The X chromosome contains the Tfm Gene (testicular feminisation gene), which directs the synthesis of a specific protein in the surface membranes of the developing reproductive system cells. This protein acts as a testosterone receptor. Because both male and female embryos possess at least one X chromosome, this protein is present in individuals of both sexes. The Y chromosome carries a gene designated as Tdy (Testis-determining gene). This gene encodes a protein called the H-Y antigen, which is present On the surface of all somatic cells in males but is absent in females. It directs The Development of the undifferentiated Cells of the genital ridges (gonads) into seminiferous tubules and interstitial cells. In the absence of the H-Y antigen, an Ovary develops from the genital ridge. Testosterone secreted by the developing testis enters the embryonic Circulatory system and interacts with testosterone receptors located in the target cells of the prospective Male Reproductive System. The resulting testosterone-receptor complex translocates to the nuclei, where it activates genes associated with the development of Tissues that give rise to the male reproductive system. Consequently, an XY embryo develops into a male fetus. The Tissues of the prospective FEMALE REPRODUCTIVE SYSTEM are neither activated nor developed. The absence of testosterone in an XX embryo allows the Development of the female reproductive system. These events are schematically illustrated in Fig. 21.53.

The localization of the testis-determining gene on the Y chromosome has now been established. It has been named SRY (sex-determining region); its length is approximately 30,000 Base Pairs (the total length of DNA in a human Cell is roughly 300 million base pairs).

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Fig. 21.53. Events occurring during Sex Determination in the embryo during the Cytology/cytology/16.html">Early stages of reproductive system development.