What do breeders call the effect when a breed resists modification?
I'm honestly not sure if one exists. They simply lack the gene/allele - the only specific term I can think of when it comes to lacking a gene is referencing Knockout variations where a researcher purposefully removes a gene from a subject in order to observe the effect.

However, "Knockout Mice/etc." imply intent on behalf of the researcher, not a result of breeding or natural lack of an allele. Perhaps "Genetically Incapable" would be a phrase meaning the same thing - but I'm not aware of any 'official' shorthand for 'Cannot because it's not in their genes.'