Through Anomalous Eyes

Wilson's Cap ISDS 3036, Most Influential Sire, CEA carrier/affected

Collie Eye Anomaly is an inherited genetic disease which causes abnormal structure within the eye.  It has variable expression from mild dysfunction to blindness and there is no treatment.  CEA is one of the few genetic diseases in Border Collies that there is a DNA test (developed in 2005) for, and thus it has gotten more attention than other endemic Border Collie diseases that do not have tests; i.e. epilepsy, exercised induced collapse, cancer, etc.

The ABCA estimates:

The incidence of CEA in Border Collies in North America is about 2.5%. The carrier rate is probably ten times that figure, or 25%.

This is consistent with a disease that has reached a stable saturation according to the Hardy-Weinberg equilibrium principle.  If we have 2.5% affected and the other conditions of equilibrium are satisfied we would expect 26.6% of Border Collies to be carriers and 70.9% to be clear.  This would lead to an affected allele frequency of 15.8% (2.5 + 26.6/2).

Although 25 in 1,000 affected isn’t as pervasive as we see with some other diseases or even CEA in other collie breeds, it has a significant penetration within the breed.

For comparison, some of the the most common single cell autosomal recessive disorders in humans are sickle cell anemia at 0.23 in 1,000; cystic fibrosis at 0.4 in 1,000; and familial hypercholeserolemia at 2 in 1,000.  Even widespread diseases like Alzheimer’s at 14.5 in 1,000 and color blindness at 13 in 1,000 in humans are half of the incidence of CEA in Border Collies.

Because CEA is a simple autosomal recessive allele, a dog needs two copies–one from each parent–to be affected, and will be a carrier if only one is inherited. The nature of this disease and the growing pool of DNA tested dogs makes it possible to trace the disease back into history and apply probabilities that a given ancestor was affected, a carrier, or clear of the disease, even though those dogs died long before DNA testing became available.

Using this method, it is 99% likely that popular sire Wiston Cap was a carrier for CEA.

Jock Richardson and Wiston Cap, Second Most Influential Sire, CEA Carrier

The ABCA acknowledges this without defaming the dog that appears on their seal:

Because some of the breed’s most notable herding dogs carried one copy of the CEA gene, the disease began to crop up when these notable dogs appeared in both the dam’s and sire’s lineage. As a result, some of the best herding dogs are carriers…

… Autosomal recessive diseases like CEA show up because people have line bred to top herding dogs which happen to carry one bad copy of that gene, eventually doubling up on it and causing affected progeny as well as some excellent herding dogs.

Although the ABCA’s Health Committee has gingerly broached the subject of line-breeding, genetic disease, and the popular sire effect; trialing culture apologist Donald McCaig sings a different tune:

I’ve heard the complaint that Border Collies aren’t any healthier than other purebred dogs, that pedigrees w/o Wiston Cap (d 1979) are rare. True about Cap. And intensive breeding to a single sire was, genetically, a risky idea. As it happens, the community dodged the bullet: Wiston Cap didn’t have anything wrong with him. And there hasn’t been another Wiston Cap – the community is “flavor of the month” and what I want in a dog aren’t necessarily the same combination of virtues and vices another equally qualified handler might want.

Well, we know that isn’t true.  We know that Wiston Cap was a carrier for CEA and we know that he has cemented his genes into the breed.

His genetic influence on the breed is 13.52%, meaning that for the CEA allele, Wiston Cap alone contributed 6.76% of the bad allele frequency.  That’s ~43% of the allele frequency we calculated above.  It’s hard to say that the community has “dodged a bullet” when one dog, not that long ago, being over bred and his descendants linebred could have single handedly accounted for 126 in 1,000 carriers and 5 in 1,000 affecteds for CEA.

The other falsehood in McCaig’s analysis is that there wasn’t another Wiston Cap and that this other popular sire was perfectly healthy.  Despite his fecundity, Wiston Cap didn’t quite reach the heights of his ancestor J.M. Wilson’s Cap (who appears 25 times in Wiston Cap’s pedigree).  Wilson’s Cap is the dog with the most genetic influence on the Border Collie breed determined by recorded pedigrees at 16.91% influence.

According to the same historical analysis that determined Wiston Cap was a CEA carrier, it’s 63% likely that Cap was CEA affected, 36% chance he was a carrier, and 1% chance that he was normal.

If Cap was affected, he alone would account for the entire frequency of CEA within border collies.  A 16.91% allele frequency would theoretically result in a Hardy-Weinberg equilibrium of 2.9% affected, 28.1% carrier, and 69% clear.  This is comparable to the estimate of the disease published by the ABCA.

If Cap was a carrier, he alone would account for 53.5% of the CEA frequency.

Applying the weighted average given to us by the historical analysis of 13.96% allele frequency, Cap would account for 88.6% of the CEA in the breed.

The two most influential sires in Border Collies both carried CEA.

Now, correlation does not prove causation, and I am not contending that CEA was a new mutation seen only in Cap and that he alone is the reason we see it in Border Collies.  What IS true is that should Cap have been affected by a brand new deleterious mutation, he was such a popular sire, so over bred and his descendants line-bred on him so often, that you’d expect to see that disease as widespread in the breed as we see with CEA.

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