With the public's eye glued to Caitlyn Jenner's every public appearance, conception about gender is swiftly changing. Still, there are many unknowns, and many concepts that push the boundaries of current understanding. Here are a few things you may not know about gender.10 Gender Differences Backed Up by Science
Peter Schattner, author of "Sex, Love and DNA," likes to tell this true story to illustrate how assumptions about gender don't always hold: "Mario" (not his real name), a perfectly healthy man, went to the doctor wondering why he and his wife weren't getting pregnant. The doctor concluded that his sperm count was zero, and follow-up appointments eventually revealed that he had two X chromosomes. "It's quite rare, but it's hardly unique," Schattner said. How can this happen? The short answer, Schattner said, has to do with a sex-determining gene on the Y chromosome called SRY. "Usually the SRY protein – which is produced from the SRY gene - initiates the male sexual-development pathway; without SRY protein, the developing embryo progresses as a female," he said. "This is just the first piece of the puzzle, though. SRY is simply a regulator protein that controls what other proteins in the sexual-development pathway are produced. One of the most important of these is called SOX9. SOX9 is not located on the Y chromosome and is the key to why Mario is male. Mario had inherited a rare mutation that altered the regulation of SOX9 so that Mario's cells produced SOX9 continuously, whether or not SRY protein was present. Consequently Mario became a man, even though he had no Y chromosome." And, because hormones also play a role in determining sex, people with "male" XY or "female" XX genes can also be intersex. In people with XX genes and a condition called congenital adrenal hyperplasia, for example, the adrenal glands produce testosterone instead of cortisol, sometimes resulting in ambiguous genitalia.Genetic 'Adam' and 'Eve' Uncovered
One of the few ways scientists have to study gender issues is through identical twins who were raised separately. In one such case, two boys raised apart each started identifying as female between the ages of 11 and 13. But there are other cases where only one twin identified as a different gender, suggesting that while biology is clearly at play, scientists have a long way to go before determining which of our 24,000 genes may be responsible for such phenomenon.Video: How Twins Advance Science
The blood of women who have given birth to boys often contains Y-chromosome gene sequences. This is thought to happen during pregnancy. But even women who don't have sons sometimes have the "male" DNA. Authors of a study on the subject speculate that possible sources of the DNA could be an unrecognized male pregnancy or unknown male twin, or sexual intercourse alone.Male DNA Found in Brains of Women
Holly Ingraham, professor of Cellular and Molecular Pharmacology at University of California, San Francisco, says that not much is known about the difference between genders in how tissues behave because "research people have mainly used male systems," she said. "Almost exclusively, most papers published will only have used male mice; most scientists use male mice to study almost everything." In other words, important gender differences could have been missed along the way. In addition to the lack of females, mice present another problem for studying issues of gender identity: You can't ask them if they feel male or female.Video: See-Through Mice Created in Lab
Much is known about what determines sex, Schattner and Ingraham agree. But surprisingly little is known about sexual preference, and even less about gender identity, they said. With new technology for sequencing genomes in place, the current holdups are finding enough people with rare conditions to study, and finding funding, they said. "The amount that scientists know now how sex and gender are determined is vastly greater than what we knew 20 years ago – and almost definitely is tiny compared to what will be known 20 years from now," Schattner said.Genders Not So Different?