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Cresting is a mutation, and is one that is indicates infertility and other, more serious issues in reproduction. A pair of crested may have a fertility rate of sub 50% in addition to producing offspring with other genetic issues.
There are some examples of breeders who have created a good stock with less issue, like Metzer's crested Pekings, but for the most part crested=bad news. It's a defect.