  We consider an existing framework of models where the neutrino Dirac and
Majorana mass terms are related, and clarify how the mass structures interact
through the type I seesaw mechanism, producing exact tri-bi-maximal mixing for
the effective neutrinos in the limit of strong Majorana mass hierarchy
(sequential dominance). In order to better illustrate this, we draw the seesaw
diagrams expanded to show the relevant details, discuss an underlying symmetry
that must be preserved by the neutrino mass terms, and diagonalise the
effective neutrino mass matrix corresponding to the sequential dominance limit.
