Genetic basis of Progressive Retinal Atrophy - The Mastiff (and Bullmastiff) is affected by a rare autosomal dominant form of progressive retinal atrophy (PRA), termed AD-PRA. This means a single copy of the mutation will cause the disease—there are no unaffected carriers. The known mutation is a T4R single nucleotide substitution in the rhodopsin (RHO) gene, which is critical for photoreceptor function in the retina. All dogs inheriting the mutation will develop some level of vision loss. A DNA test for this mutation is available, with results reported as clear or affected. Breeding affected dogs is strongly discouraged to prevent producing 100% affected offspring.
Pathophysiology - AD-PRA causes degeneration of the photoreceptor cells in the retina. In affected Mastiffs, retinal function appears normal at 3–4 months of age, but by around 13 months, electroretinography (ERG) detects abnormal photoreceptor activity. The mutation triggers focal retinal degeneration, with light exposure accelerating photoreceptor loss. In bright light, severe degeneration can occur in under a month. In dim-light environments, some photoreceptor repair may occur over weeks to months, slowing the progression of vision loss. Ultimately, this results in night blindness followed by progressive day vision loss, leading to complete blindness in variable time frames (from 6 months old to several years of age).
Complications - Complete blindness over time (speed depends in part on light exposure). No treatment or cure — management is supportive only. Increased risk of secondary eye problems such as cataracts or uveitis, which may cause discomfort. Significant lifestyle and safety considerations for blind dogs: higher risk of injury in unfamiliar or hazardous environments if not managed carefully.
Why This Matters to Breeders and Vets - Breeders: Because AD-PRA is dominantly inherited, breeding an affected dog guarantees affected offspring. DNA testing must be used to ensure only clear dogs are used for breeding in Mastiffs and Bullmastiffs. Veterinarians: Early diagnosis through DNA testing and/or ERG allows for environmental management to slow progression (e.g., reduced light exposure). Vets can also guide owners in preparing the home environment for a visually impaired or blind dog. This form of PRA is unique — most canine PRAs are recessive — so the breeding strategy and risk assessment are different from many other breeds.
Summary - Progressive retinal atrophy – dominant (Mastiff type) is a rare, autosomal dominant inherited retinal degeneration caused by a rhodopsin gene (RHO) mutation. It leads to progressive and ultimately complete blindness, with speed of progression influenced by light exposure. Affected dogs can retain vision longer in low-light environments, but there is no cure. Genetic testing makes prevention entirely possible through careful breeding. Early diagnosis and light-management strategies, combined with environmental adaptations, allow affected dogs to live safe and fulfilling lives despite eventual vision loss.