Pedigree dogs again

The Independent has a long Crufts article, in places a little odd:

When it comes to the health of breeding dogs, Carol is a little ambivalent. Clumber spaniels tend to be affected by a genetic deficiency called PDP1, which can retard the body's metabolism and make the dogs too exhausted to exercise. Many dogs carry the PDP1 gene and lead long happy lives without being affected. But should they be used for breeding? It's a question at the heart of the Kennel Club controversy. "If they were affected with the disease, you wouldn't breed with them," said Carol. "But you can use a carrier for mating purposes, provided it's mated with a clear dog, so you can breed it out."

She is a fan of the Kennel Club, who gave her a £4,500 grant in 2004 to test 100 dogs for the virus. [my italics]

A genetic defect isn't a virus and, if the comments to the article are anything to go by, a large part of the fury over the whole pedigree dog question stems from people not understanding the basic science. 

Every one of us has some deleterious genes, but most of the time nothing catastrophic results because of genetic diversity —  bad genes are rare, so it is unlikely that two parents have the same problem genes and hence their children won't often inherit two bad copies and suffer the actual disease. Cystic fibrosis is an example: the gene is recessive and 1 in 25 members of the population carry it. However the chance that a baby's parents will both be carriers is only 1 in 625 (1 in 25 x 25) so it is uncommon for children to be born with the disease.

1 in five clumber spaniels is a carrier for PDP1, so there is a 1 in 25 chance of both parents being carriers if breeding pairs are chosen randomly, and this is why health checks are so essential — and are rightly encouraged by the Kennel Club.

However: 1 in five is an astoundingly high frequency for a gene that will make you very ill if you inherit two copies. That kind of frequency would only happen under natural conditions in two possible scenarios.

1. The population size was reduced to a very few individuals at some point and, by chance, some of those individuals happened to be carriers (for example if a few animals were carried to an island on a floating log).
2. Animals with one "bad" and one "good" copy of the gene had some survival advantage; as in the case of human Sickle Cell Anaemia

In Clumbers, the most likely cause is that humans inadvertently caused an artificial analogue of the first option; either because a very small number of dogs were used to create the breed originally, or because at some point a very popular sire happened to be a carrier. 

Independent of the danger that harmful recessive genes may become abnormally frequent if some dogs sire huge numbers of puppies, inbreeding means that both a puppy's parents are likely to carry the same copies of any harmful genes. It doesn't cause bad genes, but it dramatically increases a puppy's chances of inheriting two copies and suffering the actual disease rather than being a carrier.

So - some of the furious argument about cross-bred dogs is actually a red herring, because the need for sophisticated health checks before breeding is, at least partially, an artifact of the abnormal population structure of pedigree dogs.  

The Dogs Trust/Kennel Club Independent enquiry into dog breeding now has a website.