  The idea of quark-lepton universality at high energies has been introduced as
a natural extension to the standard model. This is achieved by endowing leptons
with new degrees of freedom -- leptonic colour, an analogue of the familiar
quark colour. Grand and partially unified models which utilise this new gauge
symmetry SU(3)_\ell have been proposed in the context of the quartification
gauge group SU(3)^4. Phenomenologically successful gauge coupling constant
unification without supersymmetry has been demonstrated for cases where the
symmetry breaking leaves a residual SU(2)_\ell unbroken. Though attractive,
these schemes either incorporate ad hoc discrete symmetries and
non-renormalisable mass terms, or achieve only partial unification. We show
that grand unified models can be constructed where the quartification group can
be broken fully [i.e. no residual SU(2)_\ell] to the standard model gauge group
without requiring additional discrete symmetries or higher dimension operators.
These models also automatically have suppressed nonzero neutrino masses. We
perform a systematic analysis of the renormalisation-group equations for all
possible symmetry breaking routes from SU(3)^4 --> SU(3)_q x SU(2)_L x U(1)_Y.
This analysis indicates that gauge coupling unification can be achieved for
several different symmetry breaking patterns and we outline the requirements
that each gives on the unification scale. We also show that the unification
scenarios of those models which leave a residual SU(2)_\ell symmetry are not
unique. In both symmetry breaking cases, some of the scenarios require new
physics at the TeV scale, while others do not allow for new TeV phenomenology
in the fermionic sector.
