  We extend the nonsupersymmetric SO(10) grand unification theories by adding a
horizontal symmetry, which connects the three generations of fermions. Without
committing to any specific symmetry group, we investigate the 1-loop
renormalization group evolutions of the gauge couplings with one and two
intermediate breaking scales. We find that depending on the SO(10) breaking
chains, gauge coupling unification is compatible with only a handful of choices
of representations of the Higgs bosons under the horizontal symmetry.
