Mapping by Deletions
Deletions may also be used in genetic mapping. The recombination frequency for generating functional genes is not measured in this type of mapping. Instead, all that is asked is whether or not a deletion and a point mutation can recombine to yield a functional gene. If they can, then the deletion must not have removed the nucleotide allelic to the point mutation.
Consider a series of strains each containing a deletion or a point mutation. Suppose the point mutations lie within a gene X and that the deletions all begin beyond the left end of X and extend various distances rightward into X (Fig. 8.11). If a diploid between ∆1 and point mutation A can yield an X+recombinant, then A must lie to the right of the endpoint of ∆1. If A also fails to yield X+ recombinants with ∆2, then ∆2 ends to the right of A and hence to the right of ∆1. By this type of reasoning, a completely unordered set of deletions and point mutations may be ordered.
Figure 8.11 Recombination be-tween a chromosome containing a point mutation and a DNA frag-ment containing a deletion. Crossovers can occur only be-tween homologous segments, that is, outside a deleted area.a greater fraction of B+C+