Double muscling is extremely heavily muscled animals that appear to have 2 of every muscle. In reality, they only have one but it is much larger than normal. Belgian Blue cattle are known for this trait. It's caused by a mutation in the myostatin gene which when functioning correctly "turns off" muscle growth, per se. It's more complicated than this, but that's the easiest explanation.
The "newer" genetic defects would include AM, NH and CA in Angus. Double muscling ins't really new since the mutation has been around for a long time, but the Angus association is now paying a lot more attention to it. The polled gene isn't a defect in the sense of what we think of as a defect. To really get your point across I'd go with the lethals (AM, NH and CA). Their inheritance patterns are the same as TH and PHA (autosomal recessive) so it might be easier to explain to your audience. You'd only have to give one lesson in genetics and then can focus on what makes them lethal, why it was important to make breeders aware, etc. You can find a lot of information on breed websites. Good luck!
