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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 leptonstransforming as triplets under a new gauged $SO(3)$ isospin. In the 4deffective theory, the $SO(3)$ isospin is broken to $U(1)'$, through a$S^1/(\mathbb{Z}_2\times \mathbb{Z}'_2)$ orbifold, then subsequentlydynamically broken, resulting in a massive $Z'$. Quarks and leptons in the 5dbulk appear as massless modes, with zero Yukawa couplings to the Higgs on thebrane, and zero couplings to $Z'$, at leading order, due to the $U(1)'$symmetry. However, after the $U(1)'$ breaking, both Yukawa couplings andnon-universal $Z'$ couplings are generated by heavy Kaluza-Klein exchanges.Hierarchical quark and lepton masses result from a hierarchy of 5d Diracfermion masses. Neutrino mass and mixing arises from a novel type Ib seesawmechanism, 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 correspondingquark and lepton effective Yukawa couplings.

Quark and Lepton Mass and Mixing with non-universal $Z'$ from a 5d Standard Model with gauged $SO(3)$