Turns out, whether or not you are a ginger is not determined by the simple genetics of a single gene. In fact, the pigment that causes red hair is likely present in many brunettes. What matters more seems to be how much of the ginger-hiding brunette pigment you have — and the genetics that determine that are a lot more complicated. Which, frankly, makes the brunette-guy-with-red-beard phenomenon make a whole lot more sense.
This is only one of the many human genetics myths that get debunked at the website of geneticist John McDonald. He's dedicated to tearing apart the simple lies-to-children that most of us picked up in grade school, junior high, and high school — oversimplifications of things like tongue rolling, dimples, and eye color that make it easier for kids to understand genetics, but end up leaving adults with a skewed understanding of how genetics actually work.