I had thought the horn/polled gene was coded for on a couple of pairs of genes because you can get 4 phenotypes:
Horns
Scurs
Polled
Double-polled (little dips where the horns should be)
Lleyns, for example, are double-polled. This makes them ideal for crossing to horned breeds to produce poled offspring - the most you will is scurs if one of the parents is double-polled.
I had no idea it was possibly sex-linked too.
I have read no published papers on this - this is just what I gleaned (coupled with supposition) from a semi-scientific verbal explanation I once had. However, I was looking at sheep to buy at the time, so my ind was on that, really.