An interbred results from mating hybrids to mates of the same breeding. For example, F1 x F1. A new breed can be generated by stabilizing the interbred as a synthetic breed.
What is the effect of applying selection to synthetic breeds?
If the selection criteria favour one of the founder breeds, the selected population will become more like the founder. An example is the Corriedale which was derived from a cross between Lincoln and Merino breeds and then interbred over many generations. In many flocks, selection has been applied towards finer wool, meaning that later generations are more like Merinos. Eventually, all of the Lincoln genes could be eliminated.