Recombination within a subclass of restriction fragment length polymorphisms may help link classical and molecular genetics.

Restriction fragment length polymorphisms (RFLPs) are being used to construct complete linkage maps for many eukaryotic genomes. These RFLP maps can be used to predict the inheritance of important phenotypic loci and will assist in the molecular cloning of linked gene(s) which affect phenotypes of scientific, medical and agronomic importance. However, genetic linkage implies very little about the actual physical distances between loci. An assay is described which uses genetic recombinants to measure physical distance from a DNA probe to linked phenotypic loci. We have defined the subset of all RFLPs which have polymorphic restriction sites at both ends as class II RFLPs. The frequency of class II RFLPs is computed as a function of sequence divergence and total RFLP frequency for highly divergent genomes. Useful frequencies exist between organisms which differ by more than 7% in DNA sequence. Recombination within class II RFLPs will produce fragments of novel sizes which can be assayed by pulsed field electrophoresis to estimate physical distance in kilobase pairs between linked RFLP and phenotypic loci. This proposed assay should have particular applications to crop plants where highly divergent and polymorphic species are often genetically compatible and thus, where class II RFLPs will be most frequent.