Why a strand of hair bends or falls the way it does may sound like a simple question, but the answer is rather convoluted. On one level, the texture of a person's hair derives from his or her genes. A 2009 study looked at the genetics of waves and curls and reported a heritability of between 85 and 95 percent. (That means about nine tenths of the variation in hair texture within the sample could be ascribed to DNA.) How does this play out at the level of a single hair? Research shows that the curvature of a strand depends on the nature of its follicle. When a follicle is asymmetrical, the hair that it produces is oval in shape and tends to curl. When it's symmetrical, the strand that emerges grows round and straight.