Chromosomes are long strings of DNA that hold many genes; humans have 23 pairs of chromosomes, one set inherited from each parent. Females have two X chromosomes, while males possess one X and one Y. The new study, published in the journal Hormones and Behaviour, was done in mice, but researchers said the genes that determine sex are similar in all mammals, including humans, so the results might be applicable, especially in males with Klinefelter's syndrome, or those who have an extra X chromosome.
"Whether this is a specific phenomenon to mice, or even to this particular inbred background strain of laboratory mice, is still an open question, but we did find similar results in two different genetic models of mice," said study researcher Paul Bonthuis, a graduate student at the University of Virginia. "To know how general the finding is to other mammals one would have to do studies with other mammalian species directly," he was quoted as saying by LiveScience.
In the study, the researchers studied two special lines of mice. First, they were able to separate the effects of the X and Y chromosomes from the mouse's sexual development (which is dependent on one gene on the Y chromosome).
This means that even lab mice with odd numbers of sex chromosomes for instance males with two X's or females that have a Y still developed normally; they had normal genitalia and sexual behaviours for their gender.
In another mouse model, the researchers linked the X and Y chromosome so this pair could be matched up with an X chromosome, resulting in genetically XXY males. These two models provided different parts of the puzzle, with the first one revealing developmental differences created by genes on either the X or Y chromosome that aren't involved in normal development.
When the researchers studied these mice, they found that the males with two X chromosomes (the XXY males) were about twice as fast to ejaculate and did it nearly twice as often than those with only one. They also mounted females more often and during sex showed more pelvic-thrusting motions. "We take these findings to mean that not all sex differences in behaviour are due to the differences in the hormone secretions by the ovaries and the testes," Bonthuis said. "Our studies indicate that direct genetic differences between XX and XY individuals...also play a role in causing sex differences in behaviour."
These results indicate that there may be an undiscovered gene on the X chromosome that affects sexual behaviours in mice and perhaps in other mammals, including men, the researchers said. "We do not yet know what gene on the X chromosome is causing this effect on behaviour," Bonthuis said, but only a small per cent of genes are expressed on both X chromosomes (some genes are automatically turned off in one X when two X chromosomes are present). "It is hard to say for certain what the X factor could be, but we do have some likely candidates," they said. Human males with two X chromosomes do exist.
They have Klinefelter's syndrome, which happens about once every 500 to 1,000 male births. Symptoms include infertility and decreased testosterone levels, among others. Many males don't show symptoms. A 1997 report on men with lowered fertility indicated that men with Klinefelter's reported having sex more often than normal XY males. If this holds true, they could be under the influence of the same mysterious X chromosome factor as the mice.