Phenylalanine

- 1st ex where mutation take out function of preexisting gene: chemical named phenylalanine (has it's uses but can become toxic to neurons if too levels too high) & there is enzyme that turns phenylalanine into something safer. If mutation in gene coding for that enzyme (just a point-mutation) -> enzyme that doesn't do its job -> phenylalanine destroys nervous system via Phenylketonuria  ( PKU ) (genetic disorder). So effect is dramatic: can't reproduce bc nervous system is destroyed after birth.