Alkaptonuria is a rare inherited genetic disorder in which the body cannot process the amino acids phenylalanine and tyrosine, which occur in protein. It is caused by a mutation in the HGD gene for the enzyme homogentisate 1,2-dioxygenase.
The cause of Alkaptonuria is the fact that both your parents have a missing chromosome, this is very rare for both parents to have the same missing chromosome.