  We consider an extension of the Georgi-Glashow SU(5) GUT model by a
45-dimensional scalar and a 24-dimensional fermionic representation, where the
latter leads to the generation of the observed light neutrino masses via a
combination of a type I and a type III seesaw mechanism. Within this scenario,
we investigate the viability of predictions for the ratios between the charged
lepton and down-type quark Yukawa couplings, focusing on the second and third
family. Such predictions can emerge when the relevant entries of the Yukawa
matrices are generated from single joint GUT operators (i.e. under the
condition of single operator dominance). We show that three combinations are
viable, (i) $y_\tau/y_b=3/2$, $y_\mu/y_s=9/2$, (ii) $y_\tau/y_b=2$,
$y_\mu/y_s=9/2$, and (iii) $y_\tau/y_b=2$, $y_\mu/y_s=6$. We extend these
possibilities to three toy models, accounting also for the first family masses,
and calculate their predictions for various nucleon decay rates. We also
analyse how the requirement of gauge coupling unification constrains the masses
of potentially light relic states testable at colliders.
