The unification of gauge and Yukawa couplings within the minimal super
symmetric standard model is studied at the two loop level. We derive a
n expression for the effective scale, T(SUSY), which characterizes the
supersymmetric particle threshold corrections to the gauge couplings,
and demonstrate that T(SUSY) is only slightly dependent on the squark
and slepton masses, and strongly dependent on the higgsino masses as
well as on the mass ratio of the gauginos of the strong and weak inter
actions. Thus, if the gaugino masses proceed from a common soft supers
ymmetry breaking term at the unification scale, and there is no signif
icant gaugino-higgsino mixing, T(SUSY) will have a strong dependence o
n the supersymmetric mass parameter mu. Moreover, the value of the top
quark Yukawa coupling necessary to achieve the unification of bottom
and tau Yukawa couplings is also governed by T(SUSY), and it yields pr
edictions for the top quark mass which are close to the quasi infrared
fixed point results associated with the triviality bounds on this qua
ntity. From the requirement of perturbative consistency of the top qua
rk Yukawa sector of the theory, we obtain constraints on the allowed s
plitting of the supersymmetric spectrum, which, for certain values of
the running bottom quark mass, m(b), are stronger than those ones comi
ng from the experimental constraints on the strong gauge coupling. For
example, for m(b)(M(b)) less-than-or-equal-to 4.1 GeV, which approxim
ately corresponds to a physical mass M(b) less-than-or-equal-to 4.7 Ge
V, we obtain that, for moderate values of tan beta, perturbative unifi
cation may be achieved only if alpha3(M(Z)) less-than-or-equal-to 0.12
4.