Phenylketonuria is an autosomal recessive genetic condition caused by a mutation in the PAH gene, which can be inherited from the parents, but it can also happen from a new mutation.
Mutation can happen during the production of new reproductive cells (sperms and eggs) because there is always a chance for mutation during the replication of genetic material, such as the one that occurs before meiosis.
However, given that this is an autosomal recessive condition, which means you need two copies of the mutated gene to express the condition, is more likely that Mateo's parents are both carriers of the condition, meaning they each have a defective copy of the gene but don't have the condition.
Each of us has two copies of each chromosome, with one of these copies coming from our mother and the other from our father. Reproductive cells have only half of the genetic information because they produce by meiosis, where the chromosomes undergo a crossing over, which mixes the original chromosomes and also results in a total of four cells with a copy of each chromosome.
This means that both the egg and the sperm that originated Mateo had a defective copy of the PAH gene, that was always present in his parents, and that is how he has PKU.
