I'm no genetics expert, but here's what Chimera has to say in the breeding chapter of Cervante's "Bible".
An F1 hybrid population is obtained by crossing two unrelated, true breeding varieties. F1 hybrids are unique in that they are uniform when grown from seed, but, like all hybrids, are genetically unstable. If reproduced sexually by inbreeding within the F1 population, the subsequent generation will be neither uniform nor similar to the F1 generation.