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Thursday, February 2, 2012

Inbreeding, its a Bad Thing

The above statement may seem obvious enough, but many animal breeders practice it with alarming frequency. The topic of inbreeding (or “linebreeding” if the baby turns out okay) has been on my mind for a while now. I know it was utilized to “fix” desirable traits in many registered purebred horses, but I didn’t really ponder the frequency of inbreeding until recently. And it’s not just horses. The family tree of most purebred dogs is now a bush. A tiny, tiny bush. The BBC documentary, “Pedigree Dogs Exposed,” (http://www.youtube.com/watch?v=xJQdYG2WHoE&feature=related) really opened my eyes to the problems of severe inbreeding in dogs. These problems include; bone and joint disorders (German Shepherds, Golden Retrievers, Great Danes, many more), obsessive compulsive disorder (Dobermans), epilepsy (Boxers and others),  Mitral valve dysplasia (King Charles Spaniels), brachycephalic airway obstruction syndrome (Boston Terriers, Pugs, Pekingese), deafness (Dalmatians), Dermoid sinus (Rhodesian Ridgebacks), and many types of cancer. Wonderful. An article in this month’s National Geographic Magazine about dog traits proved to be quite illuminating. Discussing genetic disease, the article states that “because dogs have been genetically segregated into breeds developed from just a few original individuals, each breed has a much smaller set of errant genes – often only one or two – underlying the disease.” The article continues by stating “the possibilities are especially abundant for cancer, certain types of which can show up as often as 60 percent of the time in some dog breeds but only once in every 10,000 humans.” A 60 percent cancer rate? That is CRAZY! Can’t we do some outcrossing here to improve the health of dogs? Will the world end if we cross our German Shepherds with a few Belgian Malinois to boost genetic diversity? I admit, I am not a dog person, so I’ll leave it up to the dog fans to figure out what sort of breeds should be “genetically expanded” with what.
A German Shepherd show dog. Look at those back legs.
What the German Shepherd used to look like. 
Back to horses. My favorite breed, the Arabian, exhibits lots of “linebreeding” in certain horse families. Egyptian Arabians are particularly linebred. Trying to find an Egyptian Arabian without Nazeer (a popular stallion who produced other popular stallions) in its pedigree is like trying to find a skinny person in Houston – they exist, but just barely. Most linebred horses turn out fine. Others are not so lucky. SCIDS (severe combined immune deficiency disorder) crops up from time to time within the Arabian breed, as does CA (Cerebellar Abiotrophy). Now, disease and disorders will exist up in any group of animals, but inbreeding tends to increase the frequency of problems. If a mare has X disorder and is bred to an unrelated stallion without the disorder, the baby may be healthy (depending on the disorder). But if we breed that same mare with her grandfather who also happens to be a carrier of X, then the resulting foal is more likely to inherit the disease. This is an oversimplification, of course. Determining the likelihood of disorders depends upon many more variables.
Nazeer, the "Father Abraham" of Egyptian Arabians.
Let’s turn to Quarter Horses now. HERDA (Hereditary Equine Regional Dermal Asthenia) is a disease that affects this breed (and related breeds). This disease causes the skin to separate between the superficial and deep dermal layers…which means the skin stretches and sloughs off! That is nasty. The disease is caused by a homozygous recessive gene that has been traced back to ONE stallion, Poco Bueno. A horse must have two copies of the gene in order to express the horrifying skin condition. Another genetic horror story that plagues Quarter Horses is HYPP (Hyperkalemic periodic paralysis). This condition is similar to epilepsy and can be traced to another popular stallion, Impressive. This condition is dominant, so even one copy of the gene will cause symptoms. Apparently, HYPP occurs in one in fifty Quarter Horses. All of these horses can be traced back to the same stallion. Fortunately for Quarter Horses, they are the most popular breed in the world and have numbers on their side. It should be fairly easy to find outcross pedigrees to breed with. Egyptian Arabians aren’t so lucky. I’m glad Nazeer didn’t carry any of these diseases or the Egyptian Arabian would be in hot water.
Boco Bueno
Impressive
Are there solutions to these genetic problems? An obvious answer is outcrossing. Simply breed more non-related or super-distantly related individuals. Horse and dog breeders should also utilize the wonders of modern science and get their breeding stock tested for genetic disorders. All of the above-mentioned horse disorders can be discovered through DNA testing. Testing animals will cost a little bit more money, but the health of the animal is worth it. Plus, no one said horse breeding was a cheap hobby.  Rant over…for now.

2 comments:

  1. Hybrid vigor- it does a body good!

    My first horse was a descendent of Nazeer via Morafic. He was the best horse ever, incredibly healthy until his final days.

    My gelding traces back to Impressive multiple times and is n/h (but symptom free, fingers crossed he'll stay that way). My mare traces back to both Impressive (multiple times) and Poco Bueno. Fortunately for her she seems to have avoided the horrible genetic diseases. If she were ever to be bred (NOT) I wouldn't choose a quarter horse.

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  2. Yeah, Nazeer certainly sired some nice babies, especially the sons. I hope your Impressive gelding stays symptom free! Poor baby. Is he a pasture ornament or do you ride him? I've heard that that can be a dilemma for some N/H or H/H horse owners.

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