A recent study was conducted to find out what causes hair to be straight or curly.  The consensus is it really has nothing to do with the hair itself, but rather the hair follicle.

“This tiny structure guides the hair fiber up a sort of tube as it grows. The inside of the tube determines if the hair is curly or straight — ovals produce curly hair and circular tubes yield straight hair.”

Source: http://n.pr/ePai5i