There have been a few pieces in the canine press recently that have caused some confusion, certainly with my correspondents, so I thought we had better clarify them, not least with my last column. A section header ‘Mixed matings’ at the top of the last column was omitted and would have helped the start of the paragraph which started: "And by this we mean”... etc.
The confusion here was not caused by the Kennel Club’s announcement of its ban on registering merles in certain breeds, and banning certain merle matings, but with the typographical conventions operated by the canine press.
The announcement read "There are two alleles of the M gene: MM (merle) and M+ (non-merle), with merle (MM) being dominant to non-merle (M+).”
The genetic convention is that one allele is represented by one letter, but in the announcement the merle allele was represented two letters – MM. This should have been shown as M with a superscript M. The diagram below shows the correct representation.
This way of representing these alleles is the new, modern way. Had the traditional convention been followed then the confusing typographies would have been avoided as the merle allele would have simply been ‘M’, and non-merle ‘m’, and the genotypes would have been ‘MM’, ‘Mm’ and ‘mm’. Representing merle with a capital M would have indicated this was the dominant allele, and the non-merle as lower case ‘m’ would have indicated this was the recessive allele. Why does sound, established tradition have to be so disregarded by the modernists?
Most breeds do not have the merle allele, so their genotype is mm, and for those breeds that do have M, there should be no difference in appearance between MM and Mm, as M is dominant and suppresses the effect of m. However it is the interaction of the merle gene with other colour genes that leads to different physical appearances of MM and Mm, and the interaction can have different effects in different breeds.
Furthermore the effect of M seems to be greater on black pigment than on tan pigment, these being the two basic subdivisions of melanin pigment (black/brown and tan/yellow/reddish). So for example in sable collies Mm may not show a very marked effect and simply give golden patches. Such a dog may not be physically identified as a merle but genetically it is. How the KC will cope in banning merle-merle matings in this situation when the dog is genetically a merle but is not registered as a merle remains to be seen.
The reason for banning merles in some breeds where merles do not occur ‘naturally’, and banning merle to merle breeding combinations in other breeds where merle does occur ‘naturally’, is to avoid health problems associated with the merle gene.
In this regard MM and Mm may produce different reactions. Mm can give rise to wall eye (an aesthetic rather than a health issue) and deafness. When present as MM the problems of deafness, blindness and structurally defective eyes are more serious. These differences in the degree of effect have been described as potentially leading to severely handicapped homozygotes (MM) and impaired heterozygotes (Mm). It is therefore by no means certain that Mm animals are free from problems.
The assertion in the DW leader (May 25) and by Simon Parsons that the breeding of merles (MM or Mm) with tricolours (in other words non-merle mm) carries no greater risk of inherited problems than any other mating does not stand up as heterozygotes (Mm) will continue to be produced. From a health point of view it would be best if the merle allele (M) was eliminated completely.
Several correspondents have been asking me what Prof Jeff Sampson meant in his Our Dog’s column of May 11 when he said that for him the decision to breed "would depend on just how the carrier had been identified”. I can understand what he meant but others have asked me if there are different sorts of carriers. Is a carrier identified by a DNA test different to a carrier identified by pedigree analysis? The answer should be ‘no’, a carrier is a carrier no matter how it is identified.
I am sure that what Jeff meant, although I should have no need to explain this, is that if a carrier is identified by a DNA test, it means that this method will also positively identify genetically clear dogs with absolute certainty. Pedigree analysis can be used to identify carriers – they are either the parents or offspring of an affected dog – but it does not identify which dogs are genetically clear. Therefore if your personal inclination is to breed from a carrier, you need first of all to know that it is a carrier, and then you need to know if its potential mate is genetically clear.
Having nearly been adamant in the previous paragraph that a carrier is a carrier no matter how it is identified, there is a possible exception to this. The AHT maintains that between two and 20 per cent of carriers will actually be genetically affected, this proportion varying in different breeds. It does however mean that the DNA test is not 100 per cent accurate in determining carriers. On the other hand pedigree analysis is 100 per cent accurate in detecting carriers because they are automatically the consequence of an affected dog actually being diagnosed.
So a DNA test is accurate for determining genetic clears but not 100 per cent accurate in determining carriers, and pedigree analysis is accurate for determining carriers but cannot determine genetic clears. I hope this doesn’t add further confusion, but it does explain my personal position of why I will not breed from carriers; I will only breed from genetic clears determined from DNA testing, and that includes hereditary clears.
While on the subject of DNA testing I do have to take issue with Jeff Sampson on one point that he raised in his Our Dogs column of April 20. He claimed that "the official DNA testing scheme has to be the device to remove a mutant gene from a breed’s gene pool, and the DNA control scheme then ensures that the mutant gene will not be introduced”. I could say that this is a bit of typical Kennel Club spin. A testing scheme does nothing but test, tell you the result, publish the result in the Breed Record Supplement and on Mate Select, and mark the test result on registration certificates. Testing on its own will not remove the mutant gene – that will only be achieved by appropriate breeding action that is based on the test result. Breeding action that is insisted upon by the KC is the control scheme that will remove the mutant gene and keep it removed.
Andrew Smith in DW letters of May 18 accused me of "reinforcing the stigma wrongly attached to carriers” in my column of April 27. So presumably somebody else has said they carry a stigma in order for me to reinforce it? He claimed that such a stigma "encourages breeders to conceal DNA test results if not clear”. That is incorrect in the UK, as submission for DNA testing means automatic publication of the results by the KC.
Mr Smith then claims that "limiting breeding to dogs tested clear of all conditions would severely limit the gene pool”. In an earlier column (DW March 23) I was at pains to point out that this cannot be said of all breeds and reinforced this in the column of April 27 by saying, "You cannot have the same ‘one size fits all’ approach for all breeds. Those breeds with a large gene pool should have no reason to breed from carriers; those breeds with small gene pools will need to breed from carriers, but it needs to be done in a controlled way and for a limited period of time.” That message has not been taken on board by Mr Smith.
But what I really couldn’t understand was what Mr Smith meant by "conditions that have a high incidence of presentation in affected dogs” (I assume that in genetically affected dogs the condition would always be presented at some stage), nor his position when he said that DNA testing should be encouraged because it makes possible safe breeding from a carrier, and it should be mandatory for conditions that prove fatal or are permanently debilitating but not mandatory for conditions that have a low incidence of presentation, and the tests "only provide comfort and ammunition to those who would confuse the issue”. Confused? I certainly was on this.