The good N gene is stronger than the defective d gene. Therefore, if a person has at least one N gene, he or she does not have biotinidase deficiency. An NN person does not have a biotinidase deficiency at all. An Nd person does not have a biotinidase deficiency but carries the defective d gene. This is a biotinidase deficiency carrier. A dd person has biotinidase deficiency.