It comes from a cross between black (so often angus, but not necessarily) and red Simmental. The journal article I have on it says it can happen with dark reds as well as the lighter ones, but I still think the diluted ones have it happen more. It comes from a gene where when the animal is black, you get rat tail, but the red animals it doesn't show, just waits there to be passed on. If a red animal had 2 copies of the gene, that is all they would be able to pass on, and therefore, all their "black" calves would be rat tailed. If they have just one copy, it's 50/50 chance. But if you have a black simmental that looks like a normal black, then it doesn't have the gene. I'm not sure if the people breeding black Simmentals just take this as a chance when crossing black X red, and send the rat ones down the road when they get them. I know we have them periodically (in a mixed commercial herd), and we could prevent it by breeding red. But we natural breed everything and the bulls are all black now, and for us, just not worth the expense and effort to have someone out to AI one very plain commercial cow. You can get docked for the rat tails for sure, but they are still edible
The rat tail syndrome affects more than just the tail...most notably, the way the hair is made. It's short, fine, sparse and curly (and apparently really messed up under a microscope). So there goes protection, like you said from bugs, and then also from the cold. Plus they are butt ugly and while it shouldn't matter, cattle buyers are human..... How bad it is, though, can really vary. Some of ours didn't look so bad, and as such weren't so hard doing, but a few are really awful.
I think I posted a picture a while back....don't know if I want to again....like I said, kind of embarrassing looking things.
afhm....I have never seen or heard of a Char rat tail.....do you have pictures or can you describe it. Maybe we are thinking of slightly different things.