Interrupted mating technique described earlier allows only low resolution mapping. For high resolution mapping within a distance of 2 minutes, the interrupted mating is not useful, and we need to study the stable recombinants, rather than the genes transferred. For example if ade⁺and leu⁺are the two genes transferred, we can calculate leu⁺and leu^-among ade⁺(if ade⁺ is more frequent than leu⁺). The proportion of (ade⁺ leu^-)among total ade⁺ = {(ade⁺ leu⁺)+ (ade⁺ leu^-)}, will give the recombination distance between ade⁺ and leu⁺.


It has been shown that 1 minute on the map is equal to 20 per cent recombination, calculated as above.

Linear order of genes
If two genes are very close (b and c) with respect to a third gene (a), then it becomes difficult to determine the gene order (a—b—c or a—c—b), because the recombination frequencies between a and b and between a and c will be similar. This can be resolved by using reciprocal crosses : abc⁺ (Hfr) x a⁺b⁺c (F^-) and a⁺b⁺c (Hfr) x abc⁺ (F^-). In this case prototrophs will result in equal frequencies from two reciprocal crosses if the gene order is a-b-c and they will be produced in dramatically different frequencies, if the gene order is a-c-b (Fig. 12.16).