Any text you add should be original, not copied from other sources. Thus, the sex of an offspring is determined entirely by which of the male's sperm one carrying X or Y fertilizes the egg which always carries X. Such belated discovery of the child's genetic sex creates some difficult issues of gender identity. Testosterone and dihydrotestosterone Although testosterone is one of the two primary masculinizing hormones, there is evidence that it might not be the active masculinizing hormone in certain tissues. In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. In birds and most reptiles, however, it is the opposite: The conversion of the genital ridge into the bipotential gonad requires the LHX9, SF1 and WT1 genes, since mice lacking either of these genes lack gonads. Males lacking this hormone are infertile because the testes do not descend.
Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes. The stained banding reveals variations in the DNA sequence composition. Meanwhile, during fetal development, the interstitial mesenchyme cells of the testes differentiate into Leydig cells, which make testosterone. One possible model is shown in Figure If SRY actually does encode the major testis-determining factor, one would expect that it would act in the genital ridge immediately before or during testis differentiation. The most impressive evidence for Sry being the gene for testis-determining factor comes from transgenic mice. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. Hormonal Sex Determination Genes are not enough to make a male or female. This factor organizes the gonad into a testis rather than an ovary. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. Thus, it appears that the formation of the external genitalia is under the control of dihydrotestosterone, whereas Wolffian duct differentiation is controlled by testosterone itself. To produce a human male requires not only the XY chromosome pair but also an adequate level of testosterone exposure during fetal development. The sex of an animal is not always fixed for life. The general scheme of mammalian sex determination is shown in Figure Eventually, the sex cords—now called testis cords—lose contact with the surface epithelium and become separated from it by a thick extracellular matrix, the tunica albuginea. The existence of these two independent systems of masculinization is demonstrated by people having androgen insensitivity syndrome. Through accidents of chromosomal sorting meiosis during sperm and egg production, some people inherit an XXY combination, but are still male with Klinefelter syndrome. Despite the XY karyotype and the presence of testes, such individuals develop female secondary sex characteristics. However, in the absence of gonads, the female phenotype is generated. Moreover, Sox9 expression is seen in the same genital ridge cells as Sry, and it is expressed just slightly after Sry expression Wright et al. When it differentiates, it can develop into either an ovary or a testis. In males, the Wolffian duct differentiates to become the epididymis adjacent to the testis and the vas deferens, the tube through which the sperm pass into the urethra and out of the body. The mouse gene Sry also correlates with the presence of testes; it is present in XX males and absent in XY females Gubbay et al. The mouse homologue of this gene, Sox9, is expressed only in male XY but not in female XX genital ridges. Secondary sex determination in mammals involves the development of the female and male phenotypes in response to hormones secreted by the ovaries and testes. These individuals lack a functional gene for this enzyme Andersson et al. This gene is expressed in the mouse genital ridge while it is still in its bipotential stage.
Video about sex detrmination:
16 Week Ultrasound / Gender Determination
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