Rogue, everyone's favorite southern belle, is immediately identifiable because of the white streak in her hair. I believe her particular white streak is caused by a medical condition known as sexy-itis, but what causes real white streaks to appear in people's hair?
In real life, it's a condition called "poliosis." Generally, poliosis involves a single tuft of white hair, but the hair doesn't have to be on a person's scalp. Body hair, eyebrow hair, or even eyelashes can independently and anomalously turn white. The mechanics of poliosis are no different from the mechanics of any other kind of white hair; the melanocytes, structures that make melanin and color the hair, shut down and the hair grows without pigment.
There are all kinds of reasons why one patch of hair can turn white. Inflammation in the area can damage the pigmentation cells. Benign or malignant tumors can shut off the melanocytes in one area of the body. One man found that after he'd had a severe fungal infection, the hair on the affected patch of skin turned permanently white. Scientists investigated this odd kind of fungus, which seemed to specifically damage melanocytes, only to find it was the topical medication that had caused the poliosis.
The X-Men series might be right to use a white streak in the hair as a genetic signifier. There are genetic conditions that lead to poliosis, and the most common cause of the condition is piebaldism. Piebaldism is caused by a mutation of the KIT gene, which is responsible for making a signaling protein within cells. This signaling protein is key in the development of a number of cell structures, including melanocytes. The gene mutates, the signifying protein is never made, and the hair never gets any pigment-making structures.
Most of the time, people with piebaldism have exactly the white streak pictured in comics and films — the "white forelock" that begins at the forehead and sweeps back. What we don't see in films very often is the lack of pigmentation in the surrounding area. Often, the skin, eyebrow, and eyelashes under the affected patch of hair are also white.