CEA is an inherited condition. The mode of inheritance is known as 'autosomal recessive', which means that a dog carrying two copies of the abnormal gene will be affected with the condition, but that an animal with only one copy will appear normal, but still be able to produce affected puppies.
The gene which is responsible for the main sign of CEA, choroidal hypoplasia, has been identified recently by geneticists. The cause of colobomas is less well understood, and may be the result of interaction between the CEA gene and other 'modifier' genes the dog carries.
Dogs in CEA affected breeds fall into three categories. Affected dogs carry two copies of the abnormal gene for choroidal hypoplasia. Carrier dogs carry one, with one normal copy, and appear normal on ophthalmological examination. Clear dogs carry two copies of the normal gene.
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Mating clear, affected, and carrier dogs will have different likelihoods of producing affected animals. This is illustrated for various matings below. There is no sex-linkage to CEA, so the sex of the affected or unaffected parent is irrelevant, and there is no greater likelihood of a dog or a bitch pup being affected. Because genetics is a probabilistic process, a 50% likelihood of a pup being affected from a particular mating does not imply that in an eight pup litter you will necessarily have four affected and four unaffected pups!
| Breeding two clear dogs produces a litter of clear pups. |  |
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| Breeding a clear dog to an affected dog will produce a litter of carrier pups. |  |
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| Breeding a clear dog to a carrier dog will produce a litter where there is a 50% likelihood of any given puppy being clear or a carrier. |  |
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| Breeding two carrier dogs will produce a 25% chance of a pup being affected, a 50% chance of it being a carrier, and a 25% chance of it being clear of the condition. | |
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| Breeding a carrier dog to an affected dog will produce pups with a 50% chance of being carriers or affected. | |
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| Breeding two affected dogs will lead to a litter of affected pups. | |
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In an ideal world, breeders should be aiming to produce no affected (red) pups. This is possible in any breeding consisting of at least one genetically clear parent. Unfortunately, there is no way from clinical examination to distinguish a genetically clear adult from a carrier adult or even a genetically affected 'go normal' adult.
It can be seen that breeding two carrier (amber) animals together will produce some clinically affected animals (red). Matings involving affected (red) and carrier animals will produce affected and carrier pups.
Litter screening should distinguish affected from carrier or clear animals, by detecting affected pups before they are able to 'go normal'. However, carrier animals could historically only be distinguished from clear animals by test matings. By mating a clinically normal dog to an affected dog, and examining the pups.
| Test breeding between an affected dog and a clinically normal dog of unknown genetics. |  |
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In this breeding (above), affected pups are produced which would be identified at litter screening. With reference to the matings above, we can see that the unknown dog is in fact a carrier.
| Test breeding between an affected dog and a clinically normal dog of unknown genetics. | |
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In this mating, in contrast, no affected puppies are produced, so the mystery dog should be genetically clear. However, since the genetics of any given pup is probabilistic, there would still be a small possibility of the mystery dog being a carrier.
Test mating to find clear animals is a cumbersome and inconvenient procedure, which may well produce affected pups, quite the opposite of the desired outcome!
A genetic test for CEA has recently become available, and can be used to determine the genetic status of clinically normal dogs. There is limited value in testing affected dogs, as their genetic status can be inferred. The test is offered by OptiGen and details about the test can be found on their website; the testing itself can be arranged through your veterinary surgeon, as it requires a blood sample.
The results of the test can now be registered with the International Sheep Dog Society, who run the ISDS eye scheme. For more details see Control and Links.
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| "Jess" Photo © Amanda Watkins, 2006. Used with permission. |
No one really knows. It seems likely that other genes the dog carries can produce colobomas in dogs with the choroidal hypoplasia gene, even though they seem not to cause any problems where the choroidal hypoplasia gene isn't present. The gene or genes responsible have yet to be identified, and no tests are available. These genes could be carried silently in dogs which are clear or carriers for choroidal hypoplasia.
Experience from breeders over the last few decades seems to suggest that breeding from mildly affected animals tends to produce mildly affected pups. However, this is not always the case, and matings of mildly affected dogs have been known to produce pups with colobomas, at risk of retinal haemorrhage, detachments and possible blindness. See Control for more details.
