  Grand unified theories often predict unification of Yukawa couplings (e.g.,
$h_{b} = h_{\tau}$), and thus certain relations among fermion masses. The
latter can distinguish these from models that predict only coupling constant
unification. The implications of Yukawa couplings of the heavy-family in the
supersymmetric extension of the standard model (when embedded in a GUT) are
discussed. In particular, uncertainties associated with $m_{t}$ and $m_{b}$,
threshold corrections at the low-scale, and threshold and
nonrenormalizable-operator corrections associated with a grand-unified sector
at the high-scale are parametrized and estimated. The implication of these and
of the correlation between $m_{t}$ and the prediction for $\alpha_{s}$ are
discussed. Constraints on the $\tan\beta$ range in such models and an upper
bound on the $t$-quark pole mass are given and are shown to be affected by the
$\alpha_{s}-m_{t}$ correlation. Constraints on the low-scale thresholds are
found to be weakened by uncertainties associated with the high-scale.
