UNDERSTANDING GENETICS IS CRITICAL:
The beauty of a genetic test is we can avoid breeding two carriers together.
Carrier to Carrier will cause every single puppy, to have a 25% chance of being born affected, this is one in four puppies born. This is why testing for hereditary diseases which have a DNA test available for, is a must.
It is VERY dangerous to eliminate carriers especially in breeds with already limited gene pools. Doing so makes the gene pool smaller and guess what?
You are likely to get heritable problems crop up in the remaining dogs that there is no test for yet.
If we throw out carriers we eliminate so many dogs with great qualities that we need in our breeds! Carriers are SAFE when used carefully.
Everyone needs to understand that the chart is theoretical. The actual results could be completely different.
We can't throw out an exquisite example of the breed because he/she are carriers. If given a choice between two identical dogs then of course the clear is the correct choice.
This is just odds we know that.
Remember all health testing is to be used as a breeding tool. If we get rid of every dog that does not 100% clear pass everything the breed will not be around long.
Since in the border collie there are now around 10 testable diseases and carriers recorded for each of them it is a Good rule of thumb to remember that -
ONE PARENT MUST be tested DNA NORMAL for each testable disease and you will not produce an affected pup for that disease.
Collie Eye Anomaly (CEA) CEA is a congenital disorder where the parts of the eye, particularly the retinal area, do not develop normally. The severity of the disease ranges from no visual impairment to blindness. It is not a progressive disease and affected dogs normally only have mildly impaired vision. Puppies should be tested before 12 weeks of age, if possible, by a Diplomate of the Association of Canine Veterinary Ophthalmologists (DACVO) because some dogs have a mild form of the disease called "go normal", where normal tissue grows over and covers up the diseased area as the dog matures. Identification of "go normals" is important, as these dogs are affected with CEA and will produce affected puppies just as if they had full blown expression of the disease.
This disease is much more straightforward than HD in both its inheritance patterns and in our ability to control it. CEA is an autosomal recessive disorder. Autosomal means it is passed on and expressed equally in males or females. Recessive means a dog may carry a bad CEA gene and pass it on to its offspring without having the disease itself. A dog is defined as Clear if it has no bad CEA genes. A dog is defined as a Carrier if it has one bad CEA gene and one normal gene. Both the Carrier and the Clear dogs will be unaffected and will test negative for CEA in the eye exam. A dog is defined as Affected if it eye tests positive for CEA. The outcomes of the different crosses of these dogs are as follows:
Clear X Clear = 100% CEA Clear puppies
Clear X Carrier = on average, 50% Clear, 50% Carriers
Clear X Affected = 100% Carriers
Carrier X Carrier = on average, 25% Clear, 50% Carriers, 25% Affected
Carrier X Affected = on average, 50% Affected, 50% Carriers
Affected X Affected = 100% Affected
The incidence of CEA in Border Collies in North America is about 2.5%. The carrier rate is probably ten times that figure, or 25%. The problem in controlling the disease at this time is that the only way to know if a dog is a Carrier is for it to produce an Affected puppy. Since there are many unknown Carriers out there, Affected puppies will inadvertently be produced until we have a better way to test for Carriers.
The ABCA, with support from other working Border Collie groups and owners, has funded Dr Gregory Acland from the James A. Baker Institute for Animal Health, Cornell University, to develop a DNA test for CEA. This test will identify the CEA clear dogs (those with no bad CEA genes). The test is now available!