Physical throwbacks do not occur. They breed true, even with inbreeding, although inbreeding WILL cause deformaties. They do suffer very rare mental throwbacks. It's more obvious when it's a Cyantian who has gone feral because they never could feel 'at home' in their life. There are ferals out there, a number of them. Their children are raised feral, although most are capable of blending into normal Cyantian society... and it's not unusual for a Cyantian to catch a young feral who's been snooping around inquisitively instead of running like a natural feral will do.
There really has not been any extensive human/Cyantian inbreeding, a few rare cases here and there and the breeding will almost always favor the cyantian side.