Neither of my parents have blonde or light hair and neither to any of my grandparents. So why do I just have a single blonde hair?
Answers (1)
It just happens:)
People have a very wide range of hair characteristics, both between individuals and on the same body. Considering that the various hairs growing on mammals are all essentially the same thing (keratin in a shaft grown from a follicle) just altered slightly at different locations (top of head, chest, legs etc.) it makes perfect sense that within the "normal" spectrum of hair growth you will find outliers which don't fit the more regular pattern.
That's why one can find one really long eyebrow hair, or have a small bald spot in the middle of an otherwise hairy region on your leg. Myself, I've got your standard caucasian brown hair, but for some odd reason I have a patch of a dozen or so hairs on my chin which have a distinct red-head hue. These variations certainly don't happen "for no reason", I'm quite positive that they have valid underlying biochemical explanations. You should think of them as part of the normal random variation that is present at all levels of our biology, without which evolution would be impossible.