Can I still mate them?
Sure go ahead, just don't keep any with a crest. It shouldn't cause a problem if only one carries it, however I would either sell all their offspring and get different stock that doesn't have crests instead of keeping the offspring or I'd cross them to a different duck to see whether or not they carry the gene. I might even cross your grown ducks to a different one, say put the drake with a different hen and the hen with a different drake,and see which one throws crests.
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