Genetics

Autosomal Recessive

An inheritance pattern where two copies of a mutant gene (one from each parent) are required for a condition to manifest. Dogs with one copy (carriers) appear healthy but can pass the mutation to offspring.

Autosomal recessive inheritance is the most common pattern for inherited disorders in dogs. Understanding it explains why genetic diseases persist in breeds even when affected dogs are removed from breeding programs.

How It Works

Every dog has two copies of each autosomal gene — one inherited from the sire, one from the dam. For autosomal recessive conditions:

  • Clear (N/N): two normal copies; dog is unaffected and cannot pass the mutation
  • Carrier (N/m): one normal copy, one mutant copy; dog is unaffected but can pass the mutation to 50% of offspring
  • Affected (m/m): two mutant copies; dog develops the condition

When two carriers are bred, their offspring have:

  • 25% chance Clear (N/N)
  • 50% chance Carrier (N/m) — appear healthy but carry the mutation
  • 25% chance Affected (m/m) — develop the disease

Why Carriers Are the Problem

Carriers constitute 50% of offspring from carrier × carrier matings. They appear completely normal — no symptoms, no physical signs. Without DNA testing, carriers cannot be distinguished from clear dogs by examination alone.

Generations of carrier-to-carrier matings accumulate carriers silently in a breed, occasionally producing affected offspring that alert breeders to the problem’s extent.

Common Canine Autosomal Recessive Conditions

  • Progressive Retinal Atrophy (PRA) — multiple forms, multiple breeds
  • Degenerative Myelopathy (DM) — SOD1 mutation, many breeds
  • Exercise-Induced Collapse (EIC) — Labrador Retrievers
  • Collie Eye Anomaly (CEA) — collies, Shetland Sheepdogs
  • Phosphofructokinase Deficiency (PFK) — English Springer Spaniels
  • Copper-Associated Hepatopathy — Bedlington Terriers, Labrador Retrievers

Responsible Breeding Practice

DNA testing identifies carriers before breeding. Responsible breeders:

  1. Test all breeding dogs
  2. Avoid carrier × carrier matings (which produce affected offspring)
  3. Acceptable pairings: Clear × Clear, Clear × Carrier (produces no affected dogs; 50% carriers)
  4. Over generations, select for Clear dogs to reduce carrier frequency in the breed population