What makes hair curly or straight?

A Our genes determine whether our hair will be curly or straight. Africans have tightly curled hair. Chinese have very straight hair. If one cuts across the shaft of a straight and curly hair and examines the two sections under a microscope one can see that the section of the straight hair is round while that of the curly hair is oval or flat. The flatness makes the hair bend easily and so it is curly.