Aussie Genetics Fact Sheet
by C.A. Sharp
CEA is caused by a recessive gene. Other "modifier" genes probably determine exactly what defects a dog will have, but without a double-dose of this one gene of major effect, a dog will not have CEA.
This means both sire and dam must carry the defective gene to produce an affected pup. Production of a confirmed CEA puppy proves both stud and bitch are carriers. If a CEA puppy is produced, at least one grandparent on each side is a carrier, but there is no way to tell which, unless they have also produced CEA puppies.
Statistically, only 25% of the puppies from the breeding of two carriers will be affected. Carrier animals show no symptoms and at present there is no test to detect carriers. There is no known relationship between CEA and coat or eye color. CEA has been identified in other dog breeds, including Collies, Shetland Sheepdogs and Border Collies. The gene is the same in Aussies, Border Collies and Collies. It is probably the same in Shelties, though that has not yet been established. All of these breeds almost certainly share some common ancestry with Australian Shepherds.
There is no complete listing of carrier animals, however a reference file of proven and suspected carriers is maintained by the author for pedigree analysis. Additional data is always welcome.
CEA affected puppies appear normal. The defects are within the eye and cannot be detected without special instruments. Positive diagnosis can only be made by a veterinary ophthalmologist. The specific defects the examiner will note are choroidal hypoplasia (chorioretinal dysplasia), optic disc coloboma/staphloma and retinal detatchment. Both eyes will be affected but the specific defects may differ from eye to eye.
Some CEA puppies will "Go normal." Though they become clinically normal, they still have two defective genes and any offspring they produce will be carriers. It is important that all puppies be examined no later than 8 weeks of age to catch any "go normals."
CEA is present at birth and does not progress. CEA puppies behave normally. Few will be so blind that the disease noticeably affects them. CEA does not cause the puppy any pain or discomfort. Affected animals should never be bred; but if they are not blind they can live happy and productive lives.