DNA Screening
Written by T.M Heath
At the shows who are willing to permit it, a veterinary surgeon from the Animal Health Trust will come along to the show and take blood samples from the dogs. It is not well known but taking blood samples from a healthy animal requires that the vet doing this has a special project license which is issued by the Home office and any vet taking samples from healthy dogs for a project such as this without this license could be in serious trouble with the Home Office . However should your vet be required to take a blood sample for another purpose eg. premate, haematology - it is quite acceptable that he could send part of the sample for use in the DNA project. A pedigree needs to be sent in with each blood sample so pedigrees must be taken with you to the shows which are hosting the AHT vet if you are planning to volunteer blood samples but at this stage the name of the owner or breeder can be deleted should you wish to withhold this information.
There appears to be some confusion about the difference between DNA profiling and DNA screening.
- DNA profiling: Identifies parentage
- DNA screening or testing: Identifies the animals who are affected with the disease, (affecteds) the animals who have the defective gene (carriers) and the animals who do not have the affected gene (clears)
To go into the details of how the gene is found by the Animal Health Trust is fairly complicated, would take pages of writing to explain and is best left to the experts in this field of study. What we breeders need to know is how it works once the gene is found and how we can make use of the information it gives us.
Once the defective gene has been identified and the genetic screening is available eg lethal acrodermititis, a blood sample from your dog is sent to the Animal Health Trust accompanied by his/her pedigree with your name and address and the appropriate fee and you will receive a report stating whether your dog is affected, carrier or clear.
We are now planning a mating so we have the bitch DNA sampled to see if she is carrying the defective gene for eg. lethal acrodermititis and we find she is a carrier. We then choose the stud dog who may or may not be tested. However lets assume he has been tested and he is also a carrier of LAD but he is such a splendid specimen of the breed that he still has lots of qualities you wish to retain so we go ahead and use him. In the resulting litter we find an average of 25% affected, 50% carriers and 25% clear. The affecteds we cannot use. ( Using LAD as an example it has never been known for them to breed ) This leaves 75% of the litter to choose from for future breeding stock but how do we do this as they all look normal healthy bull terrier pups?. This is where the DNA testing comes into it's own. We test the litter to find out who are clear and who are carriers and are sorely disappointed to find what we think is the best pup in the litter is a carrier. However we decide to keep this pup as we now have the advantage of available DNA testing so we can arrange that the pup we keep is mated to a genetically clear dog/bitch. In the resulting litter we then have approximately 50% carriers and 50% clears. We test this litter to find the carriers and the clears for remember they all look nice healthy pups. Hopefully this time the pick of the litter is one of the genetically clear pups so that in two generations we have bred the Lad gene out. The principals are the same for whichever hereditary problem we wish to deal with. Blood samples can be used for finding more than one defective gene on the DNA chain. Where the hereditary problem is under the control of a recessive gene eg. hearts the above principals apply and it is possible to use an affected dog providing that in the resulting litter only the clears or carriers are used for further breeding programs.
Where the gene involved is dominant as in renal nephropathy (kidney disease ) it is not advisable to use an affected dog but if this cannot be avoided do bear in mind that approximately half the resulting litter will be born affected and this can be detected by DNA screening. No matter how big and healthy looking a pup diagnosed as affected by DNA screening for the kidney problem, the pup will sooner or later will go down with kidney problem and it is well to bear this in mind should you intend selling him/her.
This is the way forward for breeding in the future. Do please get involved by giving blood samples from your dogs.
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