I believe to test this theory, the F1 offspring from a mottle bred to a wc black would need to bred to mottled and bred to white crested black. If the white crest is an allele of mottle and is neither dominant nor recessive to European mottle, then I would expect to see some lightly mottled...
In my previous comment, I included examples of offspring from a white crested black polish hen that I suspected was carrying mottle. I bred the hen to a tolbunt male to test my suspicions. Some offspring hatched with mottle, and some did not. It was a small hatch, but the results were roughly...
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I am breeding bantam naked neck white crested black/chocolate/cuckoo and laced polish. Also naked neck laced bantam polish. To introduce the naked neck gene, I bred pure Polish to non-polish.
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Here are some of my results for your research...
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Gold Lace Polish hen bred to a black Turken mixed...
I'm glad you year you haven't given up! But sorry about all the setbacks.
The white one is from 1/4 turken/Ameraucana mix... 1/4 gold laced polish and 1/2 silver laced polish. She will get bred to pure silver laced polish for even better crest and lacing in the next generation! The black one...