Ask the Vet

By Dr. Ken McMillan
DTN/The Progressive Farmer Contributing Editor

Question:

We bred a heifer that came out of a TH-positive (tibial hemimelia) bull to a tested TH-free bull. We never had the heifer tested but assumed, based on her appearance, that she was positive. Our understanding was that by breeding her to a negative bull, we could not wind up with a TH-positive calf. Yet the calf has all the symptoms of TH. Can one parent pass this along, or is there a possibility the bull was mislabeled? It has always been my understanding that in order to pass along a genetic defect, both parents must have tested positive for it. This was the cow's first calf, and now I don't know whether to put her with a different bull or cull her. She was a great female, so it's a tough call.

Answer:

First, let's define what TH, or tibial hemimelia, is for our readers. This is a genetic condition where calves with TH may have any, or all, of the following symptoms: malformed or absent tibia(s), abdominal hernia, long shaggy coat, retained testicles and meningoceles (skull defects).

Your assumptions sound correct. This is a simple, autosomal recessive gene like so many of the other genetic defects we hear about. If you have a TH-positive calf, it's pretty much proof positive that both the sire and the dam are TH carriers. It's not uncommon, unfortunately, for people to pass along incorrect information on animals they're selling, whether that's done intentionally or out of ignorance.

As to your question regarding keeping this cow that is a carrier of a potentially fatal disease, genetic testing exists as a way to choose animals free of these type defects to avoid having calves affected with the same diseases. The more calves in the population with this gene, the longer it takes to remove it from the population. When you choose to keep a known, or even suspected, carrier of a genetic defect, it's your responsibility to share that information if that animal is ever going to be introduced into a breeding herd. And when you purchase animals, you should have reliable documentation that shows the animal has been tested and is free of the defect.

(VM/CZ)

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