each parents DNA comes in the form of a double helix. With each helix possessing the coding that correlates to bodily protein responsible for the expressing of certain traits. Some of these traits are recessive, while others are dominate. It is possible for each parent to posses the genes for blond hair (a recessive trait if i remember correctly) and dark hair (a dominate trait). As such when the west have kids, their is a 1 in 4 chance of said kid being blonde. Isn't genetics fun?