Ok that is a calculator flawif i understand this correctly - when you turn off mottling or just choose 1 mottling gene the bird physically is just white.
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1 or 0 copies Mo/mo
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so at least for the calculator, there needs to be a mottling expression to get the white dom bird with spots.
There should be no mottling, het. white should show some spots (or mottling shouldn't show any. For consistency.)