If you are refering to when a bull is trying to breed a cow and the penis appears to be like a corkscrew, this is usually a sign of the bull being not quite in breeding shape, either too thin or too fat. Bascially he is not fully aroused. Trying to breed too many cows in too short a period of time causes it as well. (too many cows cycling for one bull to handle).
If you are refering to a mis-shapen penis discovered during a breeding exam, it can be genetic or an injury. If it is genetic, it won't be passed on as the bull will be unable to breed.
