That was my suspicion as well.
I had someone mention to me that he thought other breeds had been bred into them so it could be a possibility that it happened at some point in the chain. Is there anything in the Isbar that could carry the rumpless gene? If my roos are not, what are the odds of chicks coming out with the rumpless gene?
I have one blue Isbar baby that has a tuft on its head! I was told they are not pure Isbars, but they look identical to all the pictures that I can find of young Isbars and the parents also have all the characteristics of Isbars. I guess I will never know.