Male sex chromosones
Other animals[ edit ] In most mammals, sex is determined by presence of the Y chromosome. It turns out that the sex determination pathway is probably more complicated and SRY may in fact inhibit some anti-male genes. However, there are cases in which testes can develop in the absence of an SRY gene see sex reversal. Hormone levels in the male parent affect the sex ratio of sperm in humans. Even so, the absence of the SRY gene or the silencing of the SOX9 gene are not enough to trigger sexual differentiation of a fetus in the female direction. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located on the Y chromosome that code for maleness.
Not all male-specific genes are located on the Y chromosome. Human ova, like those of other mammals, are covered with a thick translucent layer called the zona pellucida , which the sperm must penetrate to fertilize the egg. Hormone levels in the male parent affect the sex ratio of sperm in humans. Other species including most Drosophila species use the presence of two X chromosomes to determine femaleness. Implications[ edit ] Looking into the genetic determinants of human sex can have wide-ranging consequences. However, there are cases in which testes can develop in the absence of an SRY gene see sex reversal. The presence of Y chromosome genes is required for normal male development. Aristotle claimed that the male principle was the driver behind sex determination,  such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female. In these cases, the SOX9 gene, involved in the development of testes, can induce their development without the aid of SRY. However, In an interview for the Rediscovering Biology website,  researcher Eric Vilain described how the paradigm changed since the discovery of the SRY gene: We [are] entering this new era in molecular biology of sex determination where it's a more subtle dosage of genes, some pro-males, some pro-females, some anti-males, some anti-females that all interplay with each other rather than a simple linear pathway of genes going one after the other, which makes it very fascinating but very complicated to study. Additionally, an abnormal number of sex chromosomes aneuploidy may be present, such as Turner's syndrome , in which a single X chromosome is present, and Klinefelter's syndrome , in which two X chromosomes and a Y chromosome are present, XYY syndrome and XXYY syndrome. In these species, one or more genes are present on their Y chromosome that determine maleness. Some other insects have the X0 sex-determination system , where just one chromosome type appears in pairs for the female but alone in the males, while all other chromosomes appear in pairs in both sexes. Maternal[ edit ] In humans and many other species of animals, the father determines the sex of the child. Sex determination system Birds and many insects have a similar system of sex determination ZW sex-determination system , in which it is the females that are heterogametic ZW , while males are homogametic ZZ. Other animals[ edit ] In most mammals, sex is determined by presence of the Y chromosome. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located on the Y chromosome that code for maleness. Even so, the absence of the SRY gene or the silencing of the SOX9 gene are not enough to trigger sexual differentiation of a fetus in the female direction. Ford and his team, in the wake of Jost's experiments, discovered  that the Y chromosome was needed for a fetus to develop as male when they examined patients with Turner's syndrome , who grew up as phenotypic females, and found them to be X0 hemizygous for X and no Y. In humans, most mammals, and some other species, two of the chromosomes , called the X chromosome and Y chromosome , code for sex. For a long time we thought that SRY would activate a cascade of male genes. Humans[ edit ] Human male XY chromosomes after G-banding In humans, half of spermatozoons carry X chromosome and the other half Y chromosome. Humans, as well as some other organisms, can have a chromosomal arrangement that is contrary to their phenotypic sex; for example, XX males or XY females see androgen insensitivity syndrome. After the discovery of the testis-determining gene SRY , many scientists shifted to the theory that the genetic mechanism that causes a fetus to develop into a male form was initiated by the SRY gene, which was thought to be responsible for the production of testosterone and its overall effects on body and brain development.
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