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The Y chromosome is one of the sex-determining chromosomes in humans and most other mammals. In mammals, it contains the gene SRY, which triggers testis development, thus determining maleness. OverviewMost mammals have one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains the gene that triggers embryonic development as a male. This gene is SRY. Other genes (in addition to SRY) on the Y chromosomes of men and other mammals are needed for normal sperm production.There are exceptions, however. Among humans, some men have two X's and a Y ("XXY", see Klinefelter's syndrome), or one X and two Y's (see XYY syndrome), and some women have three Xs or a single X (and no Y, "X0", see Turner syndrome). There are other exceptions in which SRY is damaged (leading to an XY female), or copied to the X (leading to an XX male). For related phenomena see Androgen insensitivity syndrome and Intersex.Many groups of organisms in addition to mammals have Y chromosomes, but these Y chromosomes do not share common ancestry with mammalian Y chromosomes. Such groups include fruit flies (Drosophila melanogaster and relatives), some other insects, some fish, some reptiles, and some plants. In fruit flies, the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. So XXY fruit flies are female, and fruit flies with a single X (X0), are male but sterile.Other organisms have mirror image sex chromosomes: the female is "XY" and the male is "XX", but by convention biologists call a "female Y" a W chromosome and the other a Z chromosome. For example, female birds, snakes, and butterflies have ZW sex chromosomes, and males have ZZ sex chromosomes.[attachment=293]A Male ' Y' Chromosone looking typically upright and erect !!
Klinefelter syndromehttp://www.aafp.org/afp/20051201/2259_f1.jpgKlinefelter syndrome is caused by an additional X chromosome in males (47,XXY). Clinical findings are nonspecific during childhood; thus, the diagnosis commonly is made during adolescence or adulthood in males who have small testes with hypergonadotropic hypogonadism and gynecomastia. Virtually all men with Klinefelter syndrome are infertile.EpidemiologyApproximately one in 1,000 boys is born with an additional X chromosome-47,XXY, the karyotype that causes Klinefelter syndrome.This karyotype is detected at or before birth in 10 percent of affected boys, and it is found during adulthood in 25 percent of affected men. Almost all men with a 47,XXY karyotype will be infertile; Klinefelter syndrome accounts for 3 percent of male infertility. Klinefelter syndrome is common in infertile men with oligospermia or azoospermia (5 to 10 percent).ikod
But why has the females egg, never developed to pass on more of the mother's DNA, than the father's? Wouldn't one want to pass on more of their code?
Related question: Does genetic recombination occur before/during production of gametes, or does it occur afterwards, once the zygote has formed?