Adisease called phenylketonuria causes mental diseases, reduces skin pigmentation, and also affects the coloration of urine. what kind of effect is exhibited by the gene responsible for this disease?

Pleiotropic effect.

Explanation:

A gene is said to exert a pleiotropic effect when a single gene has an influence on two or more traits that may or may not be related to each other. Due to the effect on more than one phenotypic traits, any mutation in this gene will lead to a variety of symptoms.As given in the case of phenylketonuria, a lot of traits are being affected (skin color, mental health, and urine color) this shows that the gene is exerting a pleiotropic effect.