Since the PAH gene is biallelic, and there are many disease causing mutations, most PKU patients are compound heterozygotes. (paper)
Compound heterozygosity in medical genetics is the condition of having two heterogeneous recessive alleles at a particular locus that can cause genetic disease in a heterozygous state. (wikipedia)
More than 400 disease-causing mutations have been found in the PAH gene. (wikipedia)
Why is Phenylalanine hydroxylase gene considered biallelic when PKU is also caused by the presence of two different mutant reccesive alleles at a locus with more that 400 types of mutations occurring in nature?