  We emphasize that the vanishing of the CP asymmetry in leptogenesis,
previously observed for models with tri-bimaximal mixing and family symmetry,
may be traced to a property of the type I see-saw mechanism satisfied by such
models known as Form Dominance, corresponding to the case of a diagonal
Casas-Ibarra R-matrix. Form Dominance leads to vanishing flavour-dependent CP
asymmetries irrespective of whether one has tri-bimaximal mixing or a family
symmetry. Successful leptogenesis requires violation of Form Dominance, but not
necessarily violation of tri bimaximal mixing. This may be achieved in models
where the family symmetry responsible for tri-bimaximal mixing is implemented
indirectly and a strong neutrino mass hierarchy is present with the Form
Dominance broken only softly by the right-handed neutrino responsible for the
lightest neutrino mass, as in constrained sequential dominance.
