Some Aspects of Seesaw Models for Quarks and Leptons

The present work is divided in two independent parts. In the former we study the generalization to leptons, made in a SO(10) framework, of the "Fritzch-Stech" model. Its phenomenological consequences, especially in the neutrino physics, are analysed in detail. Interesting non-orthodox patterns for the neutrino mass spectrum are also predicted, giving observable signatures at the e^+ e^- experiments. The second part is devoted to the study of a class of models, based on the so-called "Universal Seesaw Mechanism", which essentially extends the idea of the "neutrino seesaw" to all fermions. At a single generation level these models have the nice feature of explaining the smallness of the fermion masses with respect to the weak scale, M _{W}, relating the smallness of the neutrino mass with the lightness of the electron. Suitable multi-generations extensions are also analysed in detail, which can account for a top quark mass of the same order of the W-mass. We have also given the predictions on the quark mixing parameters of two particular three-generation models. The (safe) suppression of the FCNC (gauge) interactions has also been checked.