Mapping is a process in which each new locus provides another tool for future studies.
As in all species, the initial development of a partial map for pea was tedious and drawn out. Once sufficient markers had been identified to develop a primitive linkage map, the major problem became identifying a technique that would expose simple, genetic polymorphism. Initially, phenotypic polymorphism was the method of choice, first selecting natural mutants and later using mutagens to induce changes in gene expression. As biochemical techniques developed, protein and allozyme polymorphism enabled a number of laboratories to map over 50 new loci coding these proteins. The advent of DNA technology permitted the resolution of all changes in DNA sequence and, thus, any genetic variation that was present between two genotypes.