"bead" on a string theory of genetics, source?
If the Cold Spring Harbor Laboratory is considered a reputable source (it should be), you can check out their Defining the gene page which has an overview of the beads-on-a-string theory, and how it was disproved by Seymour Benzer. You could also try to find Benzer's original paper, although it doesnt seem to be available online.

Related and possibly helpful:


Benzer's page on Cold Spring Harbor website
Evolution of Genes and Genes in Evolution - very interesting paper from 1959 on genes/genetics, look on the fourth page (marked 18) for the bead-on-string theory