  We develop an approach to the origin of three generations of the Standard
Model fermions from one generation in a higher-dimensional theory, where
four-dimensional fermions appear as zero modes trapped in the core of a
topological defect, and the hierarchy of masses and mixings is produced by wave
function overlaps in extra dimensions. We present a model with unbroken U(1)
symmetry where three zero modes appear on an Abrikosov-Nielsen-Olesen vortex
due to nontrivial scalar--fermion interactions.
