Red hair is a genetic mutation.
Adding on to that, apparantly the gene for Red Hair will eventually go extinct even if every red haired person were to only mate with other red haired people.
I wouldn't delve too deep into that, but it's a tidbit of information that came up in a conversation once. I couldn't vowch for it's reliability.
The human body contains several non-essential organs that actually have no use to you anymore, yet are easily capable of becoming infected or causing pain that can even lead to death, such as the appendix and the tonsils. (Although, I'm not sure if you can die from tonsilitis.)
The nails on your thumb grow the slowest out of all of the nails on your body.
Nails are also made of the same materials as your hair, which is also the same material that the horns belonging to animals are made from.
Bones inside of the human body are actually pink. The skeleton is generally shown as white though, as it's normally bleached once removed, or over time it changes to a pale/white colour.