double spur

[Perris]

If multiple spurs are dominant, as per https://sellers.kippenjungle.nl/page3.html , can anyone please explain how I have got a 3rd generation roo expressing it, when none of his ancestors presented it?

On a separate note, growing these over the last few months does explain why he's been helping himself to oyster shell as much as the hens normally do.

 

[Amer]

Multiple spurs is dominant but I believe that a double spur is a recessive trait.

 

[Skyle]

Multiple spurs (M) is incompletely dominant, a M/m+ bird usually grows only two spurs, while a M/M one could grow more.
Maybe this trait was carried by hens, so it wasn't as much noticeable? Sometimes roosters which have multiple spurs would grow a large one and, then, the other (or others) would remain much smaller, almost un-noticeable if you don't pay much attention.

There are two more genes that "give" more spurs:
Auxiliary spurs (As), it's completely dominant and birds who have it would grow a small spur (about half an inch long or a little more) above the main one.

Then, there is double spur (ds), which is recessive and chickens who have it would grow two spurs. It's not always shown in males.