This is the first time I have read this thread as I don't have sheep and possibly won't ever, but I DO have naturally bobtailed dogs. (Brittanys in the UK, Epagneul Breton in France where they came from)
Owners in our breed have become increasingly interested in breeding bobtails particularly up here ion Scotland as we cannot dock at all. There are ways round it in England, if you have a shooting licence or can prove your bitch is regularly worked on shoots (a shoot captain's certificate) and some people are just not bothering and breeding them with long tails - so we have quite a mixture of tails in the breed.
One of mine has virtually no tail at all - about one digit I reckon; his mother and my older dog have about 2 or 3 digits and my older bitch was docked at birth along with all of her litter mates so I know she doesn't carry the bobtail gene.
It basically means that if you breed a long tail to a bobtail you get about (statistically at least) 50% of each, and if you breed two bobs together that increases to 2 thirds bobs, and 1 third tailed. The added complication is that it is really a short tail gene rather than a strict bobtail one. So we can get all lengths of tail from none at all toa full tail which is about teh length of teh body.
Some breeders are afraid of the gene as a rumour got out that it was related to spina bifida and also that it was lethal - neither is strictly true. Personally I wouldn't mate my no tail to another no tail just in case, but as for the lethal part of it, all it means is that the embryos with a double quantity of the bobtail gene will not implant. They are just never viable. So effectively litters may be smaller.
A number of years a go an eminent geneticist who bred Boxers decided to have a go at introducing the bobtail gene so that he needn't dock - he used corgis - his name is Dr Bruce Cattanach and anyone who is interested in genetics and how he went about his programme can read it here -
http://www.steynmere.com/The Kennel Club is very strict about crossbreeds being introduced and usually 7 generations of breeding true to type is the maxim before allowing pups to be registered, but they accepted his bobtailed boxers a lot quicker than that.
If you have time it is extremely interesting and may help with sheep.
Annie