The basic story was that there was a medical treatment (maybe a vaccine) that made you live longer and not age, but some people had a different gene that made them revert to a primal human state and become violent.
The humans who had a successful treatment live in a secure city. Each person has a crystal light on their shirt just below the neck. It glows in different colors — red, blue green etc, and you are only allowed to date/marry and have kids with people of the same color, as this will make sure the kids won't have the bad gene. One of the people there had a black color crystal, as he was maybe part savage but not all the way.
This is what I remember from about 25 years ago.