It is common to do that to improve vigor and genetic diversity in a breed. Pekins- or other similarly genetic birds to the one at stake. It takes many generations of selective breeding to bring them back to the standards and to ensure they are breeding true again.
Rare breeds that have extremely low numbers and therefore limited gene pools are put on a critical list. It means that most of the flocks in an area ( country )are all from an original breeding flock and that there are only very small numbers of birds being bred. Cross breeding to necessarily...