We Saw That Southern Blotting of DNA from the β^A

Essay

We saw that Southern blotting of DNA from the β^A allele produces two DNA bands corresponding to fragment lengths of 1150 bp and 200 bp based on the location of two DdeI sites. In contrast, Southern blot analysis of β^S-allele DNA produces a single DNA restriction fragment, measuring 1350 bp in length, since one of the DdeI sites is altered by SNP variation. Why is Southern blotting necessary to identify the correct bands-if you know the sizes, why can't you just identify your band on a gel? Why is the location of the probe (spanning the predicted lost DdeI site)important?