  An analytic computation is given of the generation of Yukawa couplings and of
the quark, charged lepton and neutrino masses in the framework of an SO(10)
model with a unified Higgs sector consisting of a single pair of vector-spinor
$144 + \overline{144}$ of Higgs multiplets. This unified Higgs sector allows
for a breaking of SO(10) to the gauge group $SU(3)\times SU(2)_L \times U(1)_Y$
and contains light Higgs doublets allowing for the breaking of the electroweak
symmetry. Fermion mass generation in this model typically arises from quartic
couplings $16\cdot 16\cdot 144\cdot 144$ and $16\cdot 16\cdot
\overline{144}\cdot \overline{144}$. Extending a previous work it is shown that
much larger third generation masses can arise for all the fermions from mixing
with 45 and 120 matter multiplets via the cubic couplings
$16\cdot45\cdot\overline{144}$ and $16\cdot120\cdot144$. Further it is found
that values of $\tan\beta$ as low as 10 can allow for a $b-\tau-t$ unification
consistent with current data. The quartic and cubic couplings naturally lead to
Dirac as well as Majorana neutrino masses necessary for the generation of See
Saw neutrino masses.
