The horn is recessive so if they have a polled gene they appear polled, but they randomly give one gene (the cow one and the bull one, so each has only two genes for horns or poll) to the offspring.
So a horned calf can have two polled parents.
There's been a DNA break-through with the gene identification, and QLD Uni can now DNA to see if a bull is poll/poll (which means it can only give a poll gene) or poll/horn. McGregor would have the horn as a recessive as his dam was horned.
But he may randomly always give a poll, the beauty of random is we never know, and statistically it's a 50% but your 50% may be all poll while next doors may be all horned... don't you just love stats.
Will look up the bander, very interesting.