At least 20 different types of mutations in the gene for HGO are known to cause alkaptonuria.
From the nature.com
Bateson and Garrod collaborated to show that alkaptonuria indeed follows a Mendelian recessive pattern.
From the genotopia.scienceblog.com
The biochemically trained Garrod identified the condition as alkaptonuria, an exceedingly rare and essentially harmless condition believed at the time to be caused by a microbe.
From the genotopia.scienceblog.com
More examples
A rare recessive metabolic anomaly marked by ochronosis and the presence of alkapton in the urine
Alkaptonuria (black urine disease or alcaptonuria) is a rare inherited genetic disorder of phenylalanine and tyrosine metabolism. This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase , which participates in the degradation of tyrosine. ...
The name of this condition comes from excretion in the urine of alkapton or homogentisic acid, which turns black on exposure to air. It is due to a recessive gene which in the homozygous state prevents the formation of the enzyme homogentisic acid oxidase.