You mean "genera", I take it?
Among orchids there are a number of intergeneric hybrids -- though not any natural ones that I know of. Part of what may make a genus a genus (or a species a species for that matter) is will breed together as opposed to can they breed together naturally. Orchids tend to be very specific with regards to their pollinators. Those animals which will pollinate a catteleya will not be interested in phalanopsis. The intergenerics that do exist are the result of man acting as the pollinator. Even then, whether or not an intergeneric hybrid is possible depends on just how closely related to genera are. For example, Masdevallias and Draculas are each their own genus, yet they can be interbred because those genera are very close together in the "evolutionary tree". However a Catteleya cannot be interbred with a Phalanopsis -- those genera are too far apart relationship-wise.