If none of those founders carried a particular gene, then none of the people descended from them will carry that gene either, unless a very rare mutation event happens. Like, let's say all the founders had blue eyes, and nobody carried the gene for any other color. Well, then all the villagers will have blue eyes, and maybe when somebody from the outside world finally comes in contact with them, they'll refer to the villagers as "blue-eyed people". The thing is, this only goes gene by gene. So the blue-eyed villagers may have hair that comes in many different colors. The next isolated village might have people with different eye colors, but only red hair. So if you're talking about race, which characteristic determines race? Is it eye color? Hair color? Skin color? Eye shape? Hair texture? Mouth shape? Ear shape? It's not consistent at all. And what's more, no conclusions can be drawn simply from a person's skin color or eye shape. If you get into ethnicity then you can start seeing inferences, partially because of shared culture, and partially because people of the same ethnicity share some common ancestry. But "race" in the African-Caucasian-Asian sense is meaningless biologically. What is more, historically there have been very few completely isolated groups, and there are probably none existing today. People will tend to have children with the people around them, but wherever there is contact between two groups, pregancies will result. Humans haven't been isolated in different areas long enough to truly develop into different races, and genetic diversity is spread pretty evenly.