Most GUT models have some relationship between down-type quarks and leptons, that more or less agree with the observed values after running the renormalization equations. But, what about up-type quarks?
How good are GUT predictions for up-type quarks? And, particularly,
Is there some model explaining why the mass of charm is so near of the mass of the tau lepton?
In some sense, the GIM mechanism predicts the charm mass, does it? Albeit no a GUT theory.