"Biochemical polymorphism" - Color Atlas of Genetics, Eberhard Passarge

Biochemical polymorphism is detected by
laboratory methods. It results from individual
differences in the sequences of nucleotide bases
of DNA. If a difference leads to a change in a
codon, a different amino acid may be incorporated
at the corresponding site of the protein.
This can be demonstrated by analyzing the gene
product.
Subtle differences in the many biochemical reactions
between individuals was recognized
early last century as the basis of genetic individuality
by Archibald Garrod (Bearn, 1993).