  In family unification models, all three families of quarks and leptons are
grouped together into an irreducible representation of a simple gauge group,
thus unifying the Standard Model gauge symmetries and a gauged family symmetry.
Large orthogonal groups, and the exceptional groups $E_7$ and $E_8$ have been
much studied for family unification. The main theoretical difficulty of family
unification is the existence of mirror families at the weak scale. It is shown
here that family unification without mirror families can be realized in simple
five-dimensional and six-dimensional orbifold models similar to those recently
proposed for SU(5) and SO(10) grand unification. It is noted that a family
unification group that survived to near the weak scale and whose coupling
extrapolated to high scales unified with those of the Standard model would be
evidence accessible in principle at low energy of the existence of small
(Planckian or GUT-scale) extra dimensions.
