Five things you may not have known about poliosis:

¢ What it is: Actor Johnny Depp sports one in the movie “Sweeney Todd.” The Bride of Frankenstein had one, too. Blues singer Bonnie Raitt proudly displays hers. We’re talking about a white forelock or streak. The medical name for this natural tuft of white hair is poliosis, which comes from the Greek word – “pilios” – for gray.

¢ Where it happens: While the white patch occurs most often along the forehead, poliosis can involve eyebrows, eyelashes or hair anywhere on the body. It can happen anytime in life.

¢ Why it happens: This is not a disease. Most people with poliosis are healthy and experience it only because there is no pigment in the hair and skin in that area. It can be hereditary, but it also can occur with rare medical conditions, such as piebaldism, a genetic disease with single or multiple white patches of hair. Marfan’s syndrome and Waardenburg’s syndrome are other genetic disorders where this condition is noticed.

¢ When little Johnny goes gray: Gray hair in a child is uncommon and should be evaluated by a doctor. It could just be a matter of premature graying, but some early loss of hair color can be associated with thyroid disorders, vitamin B-12 deficiency and other conditions. There have been cases of some children being born with gray patches, according to the Mayo Clinic.

¢ An overnight phenomenon? While some people claim psychological shock or trauma can turn hair gray overnight, many scientists doubt it happens that quickly. Prevailing medical opinion traces rapid hair whitening over several weeks or months to a genetic autoimmune disease called alopecia areata in which T cells mistake hair follicles for a foreign substance and aggressively attack them.