Linkage and Recombination between Molecular and Phenotypic Markers
In order to study linkage relations between a molecular marker and a phenotypic marker, we may examine the restriction fragment patterns and the phenotypes of two parents that differ morphologically as well as for molecular markers. By making crosses between two such parents, we can estimate the recombination frequencies between molecular markers and phenotypic markers in F2 or test cross progeny. In Figure 40.8, we used a hypothetical example showing linkage of red and white eyes with different molecular markers in Drosophila. It can be seen that as a result of recombination, in a test cross, 15% progeny shows association of red eye with a molecular marker associated with white eye in a parent and vice versa. Therefore, it is concluded that molecular marker is 15 map units away from the marker eye colour although it has no causal relation with eye colour. Such a linkage between a molecular marker and a phenotypic marker allows identification of genetic loci at the molecular level. A tight linkage of such a molecular marker with a disease may also be used for diagnosis of the disease and for isolation of its gene. In many of our crop plants, these molecular markers are also recommended (i) as tools for identification of cultivars, and (ii) for selection 'of desirable plants in segregating populations. In view of this, RFLPs will be increasingly utilized in future.