Study: males with an extra female sex chromosome may have increased libidos

Researchers from the University of Virginia found that mice with an XXY configuration ejaculate nearly twice as fast and twice as often as male mice that have a single X chromosome. These XXY males also mount females more often and engage in more thrusting movements. Because these mice have average levels of sex hormones, researchers believe there may be a gene on the X chromosome that accounts for this supercharged sex drive in mice and possibly other mammals. However, considering that males customarily have a much greater sex drive than females, it may be that a gene on the Y chromosome is needed to activate the libido-boosting gene on the X chromosome.

For now these findings are specific to mice and possibly the group of inbred laboratory mice used in the experiment. However, a 1997 report on lowered fertility found that men with Klinefelter's reported having sex more often than XY males. While more research must be done to confirm these results, this suggests that males with Klinefelter's are under the influence of a libido-enhancing gene on the X chromosome.

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A study published in the journal, Hormones and Behavior, has found evidence of a gene on the X chromosome in mammals that may cause an increased libido in males.

Gender in mammals is determined by the sex chromosomes X and Y. An XX combination results in a female while an XY produces a male. However, some individuals are born with an XXY combination. In humans this occurs about one in every 500 to 1,000 births and is known as Klinefelter's syndrome. Individuals who have this syndrome have male genitalia, but they are infertile and have decreased testosterone levels. One might expect that these individuals also have a decreased sex drive, but the opposite appears to be true.

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