John has alkaptonuria; it "runs in the family". john and his grandmother are both homozygous for alkaptonuria. john's grandmother has two brothers and a sister. one of the brothers had alkaptonuria and his daughter is john's mother. based on this information, john's parents are:

John's parents are "Both heterozygous for the alkaptonuria gene."

Alkaptonuria (AKU) is a prototypic inherent error in the digestion of phenylalanine and tyrosine, portrayed by the failure to use homogentisic acid. AKU is caused by homozygous or compound heterozygous transformations and was likewise demonstrated that AKU is caused additionally by the obviously fractional loss‐of‐function changes, nonetheless, the heterozygous bearers of AKU are healthy.