"Some genes are pleiotropic," Peter began one night. "That is, one gene may
affect many traits. I think the metahuman gene might be a pleiotropic. It would 
make sense. If the gene is there in the body, and the ‘magical environment’ activates it, then it could trigger subtler
genetic shifts throughout the body. This way you wouldn’t have hundreds of
thousands of triggers all waiting to go off in a person. In fact, many people 
who aren’t metahuman might have many metahuman genes, but over the centuries,
their genetic branches lost the pleiotropic metahuman trigger. Even when the 
environment changed, they didn’t have the one gene needed to activate all the
other genes."