Myths and Speculations
This is a chapter taken from the book. I am including it on this website because I so often see misinformation being stated in regards to Double Merle.
Please feel free to share this page anytime you see incorrect information being shared in regards to Double Merle!!
There has long been speculation that Double Merle can cause more health issues then just vision and hearing impairments.
Issues such as; sterility, cardio problems, skeletal defects, missing limbs, skin conditions, aggression, behavioral issues, embryonic death, neurological issues, compromised kidney function, digestive disorders, allergies and early death anywhere from a few months to a few years old.
The following information is often cited and quoted as taken from this 2006 study - Clark LA, Wahl JM, Rees CA, Murphy KE (2006) Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog.
Proceedings of the National Academy of Sciences of the United States of America 103: 1376-1381.
"In all breeds, the double merle genotype can be sublethal and is associated with multiple abnormalities of the skeletal, cardiac, and reproductive systems"
The paper then cites the following three sources were this information has been taken from; - Sorsby, A. & Davey, J. B. (1954) J. Genet. 54, 425-440 -
"Ocular Associations Of Dappling Or Merling In The Coat Color Of Dogs."
- Little, C. C. (1957) The Inheritance of Coat Color in Dogs (Howell Book House, New York)
- Sponenberg, D. P. & Bowling, A. T. (1985) J. Hered. 76, 393-394
"Heritable Syndrome of Skeletal Defects in a Family of Australian Shepherd dogs."
Note the date on the two cited studies from 1954 and 1985. Reading through each paper there is no actual connection of skeletal, cardio, sublethal or reproductive problems attributed to either Merle or Double Merle. The only mentions are a couple of anecdotal reports from breeders. These are only personal accounts rather than facts or research but for some reason have been included in these research papers and then subsequently cited in more recent papers.
The Sorsby and Sponenberg papers can be found for download as PDFs at the following link from my website - https://www.merle-sine-insertion-from-mc-mh.com/pdf-downloads/
There is also the following paper pointing towards M/M as being a possible lethal combination - D. Phillip Sponenberg - 1985 - "Inheritance Of The Harlequin Color In Great Dane Dogs."
"The closest fit occurs if only a portion of the genotype M/M - H/h is lethal. The M/M genotype alone is abnormal and subvital, and may well interact with H/h to produce lethality a portion of the time. This is the hypothesis that best fits the data."
This study was done long before Merle and Harlequin DNA testing could be done and the summary based on phenotype and speculation only. We do understand now that "Harlequin" - PSMB7 mutation in the homozygous state is an early embryonic lethal. This study can also be found for download at the link provided above.
This following paper points towards reduced fertility of Double Merles upon semen collection and testing. H. Treu I. Reetz W. Wegner D. Krause - "Andrological Findings in a Merle-Breed". June 1976 https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1439-0531.1976.tb00313.x
Again an older study done on a limited gene pool and long before any type of Merle DNA testing could be done.
In conclusion, the idea that deletion of pigment from certain Merle allele combinations - "Double Merle" - can cause any types of defects other than vision and hearing impairments are based on poorly researched and out-dated sources, as well as anecdotal evidence based on limited personal experiences that are often shared and retold as fact. There are no recent studies based on DNA testing that indicate impairments, other than vision and hearing issues, are linked to the deletion of pigment from Merle. Further investigation on a scientific level would need to be done.
While working with the rescue, "Speak, St. Louis" we Merle tested 2 "excessive white" dogs both having neurological issues and suspected of being Double Merles.
Fluff shown on page 93 is bilaterally deaf and has a neurological issue that causes swelling on the brain. He was always assumed to be a Double Merle but testing has shown him to be m/M - m/266, S/S with unexplained white. His issues are not related to Merle. Gracie also shown on page 93 was bilaterally deaf and DNA tested as having "Neuronal Ceroid Lipofuscinosis".
She was also assumed to be a Double Merle but testing has shown her to be m/[Mc+]/M - m//267, S/sp.
There is one more issue I would like to address in connection to Double Merle.
Myth "An impaired Double Merle parent when bred to an m/m - Non-Merle mate can pass on hearing and sight impairments to any resulting m/M - Merle offspring. "
Fact When a dog has a vision or hearing impairment due to being Double Merle, this is associated with the combined Merle pigmentation trait only, which is not an "inherited disease or disorder". The vision or hearing impairment is not a dominant or recessive trait but directly linked to the combination of Merle alleles and the resulting suppression/death of melanocytes. In this case the impairment is a secondary issue due to Merle "genetics" and not "inherited" from both or either parent - it is the Merle allele inherited from each parent that when combined is the cause. The impairment will not be passed along to future generations as a trait of either deafness or vision impairment/blindness as the cause is not hereditary, the only genetic trait that will be passed along is one Merle allele.
This is the difference between "Hereditary" and "Genetic".
The follow linked article is a very good explanation of the differences between Hereditary and Genetic - " Hereditary vs. Genetic: How Genetics Differs from Heredity" https://blog.thrivetalk.com/hereditary-vs-genetic/
Then there is the idea that Merle on its own - such as m/M can cause hearing impairments which is based mainly on a study from 2009
- J Vet Intern Med. 2009 Mar-Apr;23(2):282-6. Epub 2009 Feb 3. "Prevalence of deafness in dogs heterozygous or homozygous for the merle allele." Strain GM1, Clark LA, Wahl JM, Turner AE, Murphy KE. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1939-1676.2008.0257.x
Conclusion from the paper - "There was a significant association between hearing status and heterozygous versus homozygous merle genotype. For single merles (Mm), 2.7% were unilaterally deaf and 0.9% were bilaterally deaf. For double merles (MM), 10% were unilaterally deaf and 15% were bilaterally deaf." This study included 153 dogs - 109 dogs were tested as m/M and from those dogs, 3 were unilaterally deaf and 1 bilaterally deaf.
It is very important to note three issues with this paper and the idea that "single Merles" have hearing issues simply due to the Merle pattern with no deletion of pigment.
#1 - there were no tested m/m dogs as a control group. There are other causes for deafness in dogs that are not related to Merle. It is quite possible that the 4 dogs with hearing impairments had genetic causes that have not yet been identified. Testing of approximately the same number of m/m dogs from Merle breeds could have shown the same percentage of hearing impairments. #2 - since no photos of dogs are provided we cannot know how much white was involved with the 4 dogs tested as single Merles. Piebald can also cause white and deafness. At the time of the study no test for Piebald was yet available. There would also be the yet unknown and untestable Whitehead to be considered.
#3 - again since no photos of the 4 single Merle dogs are provided, we cannot know if those dogs were really m/Mh which can delete pigment to white.
It is very important when reading any published research paper to first check the date of publication - what type of DNA testing was done and what was available at that time? Was the study done on phenotype alone?
Also how the study was done - how were the dogs involved chosen?
For example the following paper cites a lower incident of deafness related to Merle within the Catahoula breed as compared to other breeds represented in the paper, which has led to the very incorrect idea that somehow Merle is "different" in Catahoulas and does not have the same ability to delete pigment and therefore cause impairments.
"Prevalence of Deafness in Dogs Heterozygous or Homozygous for the Merle Allele." - G.M. Strain, L.A. Clark, J.M. Wahl, A.E. Turner, and K.E. Murphy - J Vet Intern Med 2009;23:282-286 https://www.lsu.edu/deafness/StrainMerleJVIM2009.pdf
51 Merle patterned Catahoulas were included - 25 were tested as M/m - none were deaf.
26 were tested as M/M - 1 was unilaterally deaf and 2 were bilaterally deaf.
Here are the quoted results from all dogs in the paper - "Deafness prevalence for the 153 dogs was 4.6% unilaterally deaf and 4.6% bilaterally deaf; 9.2% total were affected. For single merles, 2.7% were unilaterally deaf and 0.9% were bilaterally deaf; 3.5% total were affected.
For double merles, 10% were unilaterally deaf and 15% were bilaterally deaf; 25% total were affected."
In comparison the percentage of Catahoula results for M/M dogs are 4% unilaterally deaf and 8% bilaterally deaf; 12% total were affected.
This leads to the conclusion that Catahoulas have on average half the chance for hearing impairments as other breeds. However, dogs added to this study were on a volunteer basis. I recall when this study was being done and the few Catahoula breeders who volunteered M/M dogs. It is fairly easy to tell if a dog is bilaterally deaf or not. If the great majority of phenotypically M/M dogs offered to the study were already known to be hearing, this paper does not offer much significant information.
The paper then goes on to state - "Breeders of breeds in which the Merle allele is under scrutiny for possible exclusion from the breed standard likely were motivated to enroll hearing dogs to protect the permissibility of the Merle pattern. It is unknown if such bias led to subject participation that was not random."
All this information is clearly provided in the paper but people very often just want to see results and conclusions; they head directly to the final numbers. Those final results are subsequently quoted. However, without all the information provided in the study the results can be misleading. It is important to read the full paper and scrutinize closely all the details and results.