  We propose a theory of quark and lepton mass and mixing with non-universal
$Z'$ couplings based on a 5d Standard Model with quarks and leptons
transforming as triplets under a new gauged $SO(3)$ isospin. In the 4d
effective theory, the $SO(3)$ isospin is broken to $U(1)'$, through a
$S^1/(\mathbb{Z}_2\times \mathbb{Z}'_2)$ orbifold, then subsequently
dynamically broken, resulting in a massive $Z'$. Quarks and leptons in the 5d
bulk appear as massless modes, with zero Yukawa couplings to the Higgs on the
brane, and zero couplings to $Z'$, at leading order, due to the $U(1)'$
symmetry. However, after the $U(1)'$ breaking, both Yukawa couplings and
non-universal $Z'$ couplings are generated by heavy Kaluza-Klein exchanges.
Hierarchical quark and lepton masses result from a hierarchy of 5d Dirac
fermion masses. Neutrino mass and mixing arises from a novel type Ib seesaw
mechanism, mediated by Kaluza-Klein Dirac neutrinos. The non-universal $Z'$
couplings may contribute to semi-leptonic $B$ decay ratios which violate $\mu -
e$ universality. In this model such couplings are related to the corresponding
quark and lepton effective Yukawa couplings.
