autosomal recessive



Over the last few years great strides have been made in understanding the genetic causes of many disease which affect dogs.

Perhaps surprisingly, many diseases are cause by single gene mutations. A dog gets one copy of each gene from its dam, and one form its sire. Most of these mutations only cause disease when a dog has two ‘bad’ copies of the gene. This is described as being an autosomal recessive trait. You will have learned about these in O level/GCSE biology (remember the wrinkled and smooth peas???), but it might have slipped your mind since then, and there certainly seems to be a lot of confusion.

For a specific mutation a dog can be ‘clear’ (two good copies); ‘carrier’ (one good copy and one bead copy); or ‘affected’ (two bad copies).

Clear and carrier dogs will NEVER show any signs of the disease in question. But if a carrier dog is bred from there is a 50:50 chance that each puppy could get a ‘bad’ copy of the gene. That’s OK if the other parent is clear…but if the other parent is also a carrier there is a 1 in 4 risk for each puppy that it will get a ‘bad’ copy from each parent and be ‘affected’.

Just to add to the confusion, ‘affected’ dogs may also appear outwardly normal, at least for some of their lives. Some conditions are invariably fatal, others may show varying severity in affected dogs, or not show up until later in life. But, if ‘affected’ dogs are bred, every puppy will get a ‘bad’ copy of the gene.

This table provided as nice summary, but remember…the way probability works mean each puppy has the discussed risk. A clear to carrier mating to result in all clears, all carriers, or a mixture. Each puppy has a 50% chance of being a carrier.

simple recessive

The status of a dog can be determined through DNA testing. A blood sample or cheek swab is taken, and the dog’s identity confirmed (by a vet or vet nurse usually). If a dog has two ‘clear’ parents it will be described as ‘hereditary clear’. The same mutation spontaneously recurring is VERY unlikely, but some breeders test again every few generations to guard against human or lab error.

Many results are recorded by the Kennel Club and can be seen on a dog’s Mate Select profile. For emerging problems breed clubs may hold a list, or breeders may test for their own peace of mind. Always ask to see the test results if they are not listed on the KC or breed club website.

A final piece of terminology you may see is ‘carrier by progeny’. This isn’t used officially but is used to denote a dog which has produced carrier or affected offspring but hasn’t been tested itself (perhaps as it has died or been sold). A dog can be ‘carrier by progeny’ either if a mating between two untested dogs produces affected puppies, or where a untested dog has been mated with a ‘clear’ dog and pups have been tested as carriers.

The great thing about DNA tests is they allow breeders to slowly remove unwanted genes, whilst keeping as many dogs in the gene pool as possible.