  We propose a simple but suggestive seesaw model with two phenomenological
conjectures: three heavy (right-handed) Majorana neutrinos are degenerate in
mass in the symmetry limit and three light Majorana neutrinos have the
tri-bimaximal mixing pattern $V^{}_0$. We show that a small mass splitting
between the first generation and the other two generations of heavy Majorana
neutrinos is responsible for the deviation of the solar neutrino mixing angle
$\theta^{}_{12}$ from its initial value $35.3^\circ$ given by $V^{}_0$, and the
slight breaking of the mass degeneracy between the second and third generations
of heavy Majorana neutrinos results in a small mixing angle $\theta^{}_{13}$
and a tiny departure of the atmospheric neutrino mixing angle $\theta^{}_{23}$
from $45^\circ$. It turns out that a normal hierarchy of the light neutrino
mass spectrum is favored in this seesaw scenario.
