Molecular Mapping of Plant Chromosomes

Abstract

Genetic maps have been constructed for a number of plant species. For maize and tomato, two of the better studied species, the maps are comprehensive and include several hundred markers each (O’Brien, 1986). It is an arduous task to construct a genetic map and their existence is a tribute to the dedication of the geneticists and breeders who have made it their labor. The techniques of molecular biology have recently offered a more rapid approach to generating genetic maps and in some ways these maps may prove to be more useful than their classical counterparts. The technique, often referred to as restriction fragment length polymorphism or “RFLP” analysis, involves cloning unique sequences of DNA from the nuclear genome. These clones are then used as radioactive probes to detect homologous sequences in plant DNA which has been cut with various restriction enzymes, separated on agarose gels and blotted onto nylon filters. Alleles are identified by differences in the size of the restriction fragments to which the probes hybridize (Fig. 1). The segregation of RFLP markers can be monitored in progeny from controlled genetic crosses and, by using standard genetic analysis, linkage groups can be constructed.