What causes phenylketonuria is caused by a mutation in chromosome 12. For a child to have this mutation, both the mother and father must have and pass on the defective gene. It is possible for a parent to have the defective gene, but not have the disease. Most often, PKU is passed to children by parents who are carriers of the disorder.
Slide show of what causes it and how it's caused:
http://www.yourgenesyourhealth.org/pku/cause.htm
Slide show of what causes it and how it's caused:
http://www.yourgenesyourhealth.org/pku/cause.htm