FAQ: can we have DF hens in SL (incomplete) dominant mutations?
A sex linked (incomplete) dominant mutation is located on the Z chromosome and is always dominant over the wild type. This mutation can be both single and double factored in males because they have two Z chromosomes. A female on the other hand has only one active Z chromosome (and a W chromosome) and can therefore never be DF, only single factored.
Keep in mind, if the mutation is SL incomplete dominant we could have different phenotypes. In the SL SF males there is still an unmutated active Z chromosome present. Since the mutation is incomplete dominant, the unmutated gene will attempt to correct the ‘error’ on the other chromosome as much as possible. Nature has been programmed to repair the errors if possible. As such the unmutated Z chromosome will try to counteract and possibly block the function of the mutated gene. So the expression of an SF bird (male) can vary from 1 until 50 percent or even more. This explains the large diversity in SF males. A SL DF male will show the complete expression of the gene (mutation). In a female there is no second undamaged gene present. So this SF bird will have a nearly complete expression of the gene. This can be misunderstood by aviculturists and they might think these females are DF. However this is not the case.