Talk about the metabolic diseases caused by the gene mutation.

Due to enzyme gene mutation. Example (phenylketonuria PKU result in mental retardation, seizured, behavorial problems, autosomal recessive allele, inability to convert digested amino acid phenylalanin into tyrosine, whom differs by OH group. This reaction is catalyzed by phenylalanine hydroxylase, PKU is a mutation in this enzymes gene, which leads to phenylalanine over dosage which leads to toxicity.