ABSTRACT
The discovery that allelic forms of enzymes (isozymes) could be separated electrophoretically on gels and detected with histochemical activity stains heralded the introduction of molecular marker technology into the field of genetics (Smithies, 1955; Hunter and Markert, 1957). With these technologies, it was no longer necessary to have a visible change in the phenotype of the organism to identify a marker locus. This significantly increased the number of markers identifiable in genetic material and made possible the production of highly saturated genetic maps for use in marker-as-sisted breeding, gene transfer, and genetic manipulation of crop species. These marker systems have now become the major tools for genetic analysis. Depending on the molecular technology employed, the markers can be highly abundant, phenotypically neutral, and detectable at early stages of growth and can show no environmental effects on detectability. These markers have been employed for deoxyribonucleic acid (DNA) fingerprinting, for construction of genetic linkage maps, for tagging of genes controlling certain traits, and as molecular landmarks for map-based cloning of genes.
