Flavor-changing neutral currents and top-quark properties in Standard Model extensions

The structure and the generation of flavor-changing neutral currents are considered in a Standard Model extension involving a singlet quark of the up type. Anomalous top-quark properties caused by singlet-ordinary mixing is described in a phenomenological way. It is shown that, for single top production and for asymmetry in electron-positron annihilation, there is an enhancement in relation to predictions of the Standard Model.