Frequently asked questions
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Does inbreeding cause inherited disease?
The gene mutations that result in inherited disease occur at random and are fairly rare events. We know from experimental data that certain chemicals and certain kinds of chemicals can cause DNA damage resulting in the mutation of the gene involved. Some of the inherited diseases that we recognise today will have resulted from such insults, but probably most result from errors in copying DNA that have gone undetected. So, inbreeding per se does not cause genetic mutations, as far as we know. However, we have already seen that many of the inherited diseases in the dog result from recessive mutations; an affected dog will have two copies of the recessive mutation. However, the carrier dog, that carrying one normal gene copy and one mutant gene copy, will be clinically normal. Inbreeding to such carrier dogs will result in a rapid build up in the frequency of the mutant gene such that eventually affected dogs will be produced. Inbreeding therefore does not cause a mutation that results in an inherited disease, but once such a mutation has occurred, inbreeding will increase the frequency of the mutant version of the gene in the breed far quicker than other more random breeding programmes.
How is a DNA test developed for dogs?
Being able to develop a DNA test for the mutation that results in an inherited disease relies on being able to get a handle on the mutant gene. Ideally, the mutant gene needs to be identified. This can be achieved in a number of different ways. The most straightforward is to use something known as the candidate gene approach. This relies on using information from species other than the dog. Because of the nature of research into inherited disease we have a far greater understanding in species like man and mouse. Often, a canine inherited disease will be identical to a disease in man or mouse and if the mutant gene has been identified in either of these species researchers can immediately test whether the same gene is involved in the canine disease. Sometimes it is and we have a very rapid route to identifying the mutations that cause inherited disease in the dog. The mutant gene that causes progressive retinal atrophy in the Irish Setter was identified by this candidate gene approach.
If no suitable candidate genes can be identified for a canine inherited condition, then the process of identifying the causative mutation is much more protracted. The usual approach will be to collect together pedigrees where one or more members of the family have been affected. The next sage is to take blood samples from members of the family. Ideally blood needs to come from the affected dog(s), its siblings, it dam and sire and its grandparents. DNA can then be made from individual blood samples. This DNA can then be analysed using special DNA markers that have been developed as part of the Canine Genome Project. The markers chosen identify unique regions along each and every one of the canine chromosomes. The idea is to try to identify a marker(s) that is always inherited by the affected dogs, but not always inherited by the clinically normal dogs. This co-inheritance of the DNA marker with the disease signifies that the marker is physically close to the mutant gene that causes the disease on one of the chromosomes. The marker is said to be linked to the disease gene. Since the markers used have all been mapped to their unique location on one or other of the chromosomes, identifying a linked marker will identify a relatively small region of just one chromosome where the mutant gene will be found. Scientists can then scan this region for potential candidate genes that can be screened for their involvement in the disease.
The identification of linked markers can lead to the development of a DNA test for the disease gene. Not surprisingly, these tests are called linked marker tests. The presence of the linked marker is diagnostic for the presence of the associated mutant gene. Linked marker tests are rarely 100% accurate because you are not directly measuring the presence or absence of the mutant gene; inaccurate diagnoses can be made if, on the rare occasion, the mutant gene and the linked marker become separated during meiosis. Identification of the mutation responsible for an inherited disease allows the development of a 100% effective test based on the identification of the causative mutation. Such gene-based DNA tests will permit an individual dog’s DNA to be analysed to determine how many copies of the mutant gene it possesses; a dog with two normal versions of the gene will be genetically clear, one with one normal version and one mutant version will be a carrier and one with two mutant copies will be affected (for a disease that results from a single recessive mutation).
DNA tests for new diseases are steadily being produced by research groups around the world and allow individual dog’s to be tested. All that is required is a source of DNA from the dog, usually either a small blood sample or a tissue scrape from the inside of a dog’s cheek. Either tissue can be processed to produce small quantities of the dog’s DNA, which can then be analysed.
What DNA tests are available for dogs?
New DNA tests for inherited diseases in the dog are steadily increasing, as our understanding of canine genetics gets more detailed. A current picture of what is available and where the tests can be downloaded here (PDF format).
Is there a DNA test that can tell me what breed my dog is?
Unfortunately, the answer to this question at the moment is ‘No’. However, there are several on-going research projects that may well reveal information that might allow such DNA tests to be developed in the future. So watch this space.
What is DNA profiling and how is it different to DNA testing for disease?
A DNA profile is a special DNA signature that is present in each of our dogs and serves to uniquely identify the dog from a small sample of its DNA. If you like, the DNA profile represents a ‘biological microchip’ that can be used to uniquely identify a dog from its DNA. However, the profile can also be used to verify the biological parentage of a dog. This is because each dog inherits half of its DNA from its dam and the other half from its sire. This means that half of the individual components that make up the DNA profile are inherited from the dam and half from the sire. Comparison of a pup’s DNA profile with that of its presumed dam and sire will verify whether they are its biological parents, or exclude one or other if one of the parents is not correct. At the moment the DNA profile does not give us any information on the genes that the dog carries, so it cannot be used to identify any disease mutations that a dog might be carrying. However, the technology that lies behind DNA profiling is very similar to the technology that is used to develop DNA tests for the presence or absence of gene mutations that are responsible for inherited disease. It really isn’t too much of a stretch of the imagination to predict that in the near future DNA profiles in the do will expand to not only give information that will uniquely identify the dog, verify or otherwise a dog’s parentage but also give information on some of the disease genes that a dog might be carrying.