Medical Genetics Because we inherit two copies of the gene for the enzyme, both must be defective to produce the disease. A laboratory test that measures how quickly an injection of phenylalanine is removed from the blood can distinguish a person who has one pku gene from a person who ha las none but the person with one is perfectly healthy because the unmutated allele produces enough of the enzyme However, these heterozygous individuals are carriers of the diseaseMedical Genetics Because we inherit two copies of the gene for the enzyme, both must be defective to produce the disease. A laboratory test that measures how quickly an injection of phenylalanine is removed from the blood can distinguish a person who has one PKU gene from a person who has none, but the person with one is perfectly healthy because the unmutated allele produces enough of the enzyme. However, these heterozygous individuals are "carriers" of the disease