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Mirrors (2) isomers

A antiquity, scientists are still far from suspecting the existence of chromosomes and various theories flourish. In particular, Aristotle suggests that summer is more conducive to the development of a male heir and declares that the more passion is burning, the more likely to give birth to a boy are high. It places the difference between the sexes in opposition of cold and hot. He defends the woman is a man "unfinished" precisely because of his coldness he considers as a brake on development of male attributes. The model of the "temperature" persisted over the centuries and comes into its own during the Renaissance. The man and woman are defined by a series of oppositions: cold / hot, imperfect / perfect, wet / dry, indoor / outdoor. Under this concept, the sex of the woman is naturally a male reversed that keeps the cold inside the body. Not until the Enlightenment that man and woman were described from a purely anatomical point of view.

A little history
The year 1890 begins a new turn when Hermann the German biologist Henking isolated chromosome X. A decade later, Clarence E. McClung moved that this item "accessory" to the function so misunderstood, is bound to the male sex. Shortly after, Biologist Nettie M. Stevens and Professor Edmund B. Wilson, both American undertake to elucidate its role. Each of their hand, they published in 1905 similar findings: in some insects, sex is determined by the presence of a small chromosome in males and large - in this case X, in that femelles.Conscients their discovery that will shake the school their faces and that she subscribed to the influence of the environment, they continue in subsequent years to search for confirmation of the initial results. The historical irony is that their main opponent sets a little later, their theory jour.C is he who finally bring an illustration undeniable chromosomal sex determination, and thus leave his name etched in the History of Sciences, discoveries were made in 1912. Hans von Winiwarter finds that the woman has two X chromosomes while men only carried one. As for Y, long gone unnoticed because its small size, light on his life is made much later. American zoologist Theophilus S. Painter, who found in several animals and humans in the early twenties.

The XY pair
This is certainly not surprising that X and Y are not jointly identified. They were the most mismatched couple. If indeed our chromosomes associate in pairs by similar credo "that likes attract", X and Y are the exception. X is sized perfectly "consistent" with that of other chromosomes, while Y is surprisingly short. Their content also differs singularly. X shows about 1100 genes, while a meager 76 Y genes. Yet they share the same origin and there was a time when they looked strangely. Evolution has, however, largely retracted Y along its length and this to some 300 million years. A degradation process that seems slow but inexorable. "Y" would be threatened? The male is it endangered? ... Scientists do not only seriously worry the fate of the chromosome but also question the sustainability of the human species ... (1)

But Who decides what is sex?
The human being has the nucleus of each cell 46 chromosomes, 22 pairs of autosomes, numbered 1 to 22 and a pair of sex chromosomes or gonosomes called X and Y. The woman has two X chromosomes while men have one X and one Y. In women one of two X chromosomes is inactivated in the form of a mass of heterochromatin, or sex chromatin, the Barr body . Thus one of the two X fonctionnel.La presence of a Y chromosome induces the development of male gonads (testicles), while its absence leads to the formation of feminine gonads (ovaries). So This is not the number of sex chromosomes affecting sex determination but rather the presence or absence of the Y chromosome . However the Y chromosome is not sufficient by itself to form an organ as complex as a testis, however, its role is vital because it contains a gene might be termed "switchman" or "master gene" that is named SRY (*) (sex determining region Y gene). This small gene (one exon), located on the short arm (Yp) of the Y chromosome, is expressed in the precursors of Sertoli cells and controls the expression of many genes located on other autosomes and the level of sex chromosome X. In the absence of SRY in the presence of two X chromosomes, the gonad differentiates into the ovary.
This period of gonadal differentiation occurs in men between the 5th and 8th week of gestation. (4)

In October 2006 The work led by the Italian Giovanna Camerino reveal some puzzling observations. In an Italian family, several brothers XX. Although sterile, each is equipped with all the advantages men. Other men had previously XX already attracted attention and it was then discovered that they had inherited the gene SRY father following his transfer from Y to X. But the Italian brothers do not SRY. How is it then that they were "male"? By studying siblings, the researchers conducted a double discovery: a protein known to RSPO1 is absent from these brothers, following a modification of its gene, then it seems essential in determining a woman's sexual.
Very rare abnormalities affecting the sex chromosomes, however, are diverse and generally make those sterile victims. Another example, mirrored in XX men, XY individuals appear to be women because they lack the SRY gene. More confusing still are the people who inherit an abnormal number of sex chromosomes. Among them, we will retain those affected by Turner syndrome or Klinefelter's one more common than others. Living with Turner syndrome women X0 - either with a single X and Y without Equipped with a uterus and ovaries reduced, they are often small and do not develop breasts or hair. In contrast, Klinefelter syndrome XXY men are affected. Children rather large, and atrophied testes remain sterile at puberty while the mammary glands can sprout. These disorders detected in childhood fortunately can be corrected at the age of puberty with hormone therapy. (1)


From male to female

known since 1950 (the work of Alfred Jost (2) ) that secondary sexual differentiation (phenotypic sex) versus primary sexual differentiation ( gonadal sex) depends mainly on hormonal factors. A. Jost showed that castration of a male embryo sex chromosome (XY) induces the development of the female phenotype.
The female genital tract is different, for He spontaneously towards women in the absence of impregnation hormonale.On said that the female sex is sex Constitution sex or "default" . In fact, three hormones involved in the masculinization of the fetus: testosterone, AMH or the Anti-Müllerian Hormone (whose gene is located in humans on chromosome 19) and insulin-like factor : insulin-like hormone 3 or INSL3 (produced only in Leydig cells) Any
actually begins in the uterus after fertilization. Although the embryo is XX or XY already, it is neither male or female until the sixth week of pregnancy. He has no ovaries or testes, but then undifferentiated genital glands. However it has a dual genital ducts: the Wolffian ducts and Mullerian ducts that evolve, first to the male line, and the second female to track ..
Under the effect of AMH, the Mullerian ducts regress, and in the presence of INSL3, have triggered the descent of the testicles into the scrotum.
interstitial cells of Leydig (sitting in the testes) secrete increasing amounts of testosterone in the seventh week, which reach their peak during the second quarter, the decisive period of male sexual differentiation. (3)

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(*) Although the action a priori dominant SRY , abnormalities of sexual differentiation have shown the involvement of other genes on the X chromosome , as well as some autosomes (9, 11, 17, 19).
See also: Role of SRY
(1) "E very ♂ or ♀, that is the question " by Séverine Altairac
(2) French biologist Alfred Jost
(3) Embryology.
c. (4) Sex determination

Read:
Men, women, the construction of difference of Françoise Héritier