Threshold enhancement and the flavor-changing electromagnetic vertex

We analyze the evolution of the flavor-changing electromagnetic vertex as a function of the external initial quark mass ${\mathit{m}}_{\mathit{e}}$, in the three-generation standard model. We find a substantial threshold enhancement in the vertex, to one-loop order, when ${\mathit{m}}_{\mathit{e}}$ is of the order of the mass of the W boson. We comment on QCD corrections.