Discrete quark-lepton symmetry need not pose a cosmological domain wall   problem
  Quarks and leptons may be related to each other through a spontaneously broken discrete symmetry. Models with acceptable and interesting collider phenomenology have been constructed which incorporate this idea. However, the standard Hot Big Bang model of cosmology is generally considered to eschew spontaneously broken discrete symmetries because they often lead to the formation of unacceptably massive domain walls. We point out that there are a number of plausible quark-lepton symmetric models which do not produce cosmologically troublesome domain walls. We also raise what we think are some interesting questions concerning anomalous discrete symmetries. 
H. Lew and R.R. Volkas