  We discuss neutrino masses and mixing in the context of seesaw type I models
with three right-handed Majorana neutrinos and an approximately diagonal Dirac
sector. This ansatz is motivated by the idea that the flavor structure in the
right-handed Majorana masses is responsible for the large mixing angles,
whereas the small mixing angle $\theta_{13}$ originates from the Dirac Yukawa
couplings in analogy to the quark sector. To obtain $\theta_{13} \approx 0.15$
we study a possible $SU(5)$ grand unified theory realization with a $U(1)
\times \mathbb{Z}'_2 \times \mathbb{Z}''_2 \times \mathbb{Z}'''_2$ flavor
symmetry and include a complex perturbation parameter in the Dirac mass matrix.
The consequences for $CP$ violating phases and effects on leptogenesis are
investigated.
