In 1909, A.E. Garrod
described certain hereditary diseases which cause certain defects in metabolism. The three diseases, namely (i) alcaptonuria, (ii) phenylketonuria
and (iii) albinism
result due to block in the metabolism where phenylalanine is broken down and utilized. These were discussed in Mutations : 2. Biochemical Level (Biochemical and Microbial Genetics)
in some detail.