The Merle Gene in Dogs: Genetics, Health Risk and Welfare

1. What Merle Is, Genetically

1.1 The Mutation

Merle results from the insertion of a short interspersed element — a mobile piece of DNA roughly 253 base pairs long — at the boundary between intron 10 and exon 11 of the PMEL gene, formerly designated SILV or PMEL17, on canine chromosome 10 (Clark, Wahl, Rees & Murphy, 2006).

This is worth stating precisely, because merle is frequently described as a point mutation. It is a retrotransposon insertion, and that structural difference is what produces the variable behavior described in the next section.

1.2 What It Does to Pigment

The insertion disrupts melanosome function and thereby the distribution of eumelanin — the black and brown pigments — in coat, skin and eye. Areas dominated by pheomelanin, the red and yellow pigments, show little or no visible merle patterning.

The practical consequence is immediate: merle is conspicuous on a black dog and can be nearly invisible on a red one. Coat color interacts with the genotype to determine what an observer sees, which is the first of several reasons appearance is a poor guide.

1.3 Incomplete Dominance

Merle is often described as simply dominant. It is more accurately incompletely dominant. One copy produces the pattern; two copies do not produce more pattern but a qualitatively different and considerably higher-risk phenotype.

That distinction is not academic. It is the entire reason merle-to-merle pairings are treated as a welfare issue while merle-to-non-merle pairings generally are not (a case where genetics rather than behavior determines outcomes).

1.4 Merle Is Not a Disease Status

A merle dog does not have a condition. It carries a genetic variant that alters pigment distribution, and most merle dogs live ordinary lives with intact hearing and vision (what a dog can actually see). The risk attaches to particular allele combinations, above all two consequential variants in the same animal, rather than to the pattern.

The distinction is not a softening. A dog described as "a merle" has been given a coat description; a dog described as carrying two long variants has been given the information that matters, and only the second is a statement about risk.

1.5 Blue Eyes Prove Nothing

Blue or partly blue eyes are common in merle dogs and are regularly read as a warning sign. The hearing data contradict that directly: eye color showed no significant association with hearing status, and neither did sex (Strain et al., 2009).

A blue-eyed merle is not thereby at higher risk, and a brown-eyed one is not thereby clear. Both statements follow from the same finding, and the second is the one more likely to mislead a buyer.

1.6 Why the Genetics Is Worth Understanding

Because every practical recommendation in this article follows from it. The instability of the insertion explains the mosaicism, the mosaicism explains why appearance is unreliable, and that explains why testing is necessary rather than optional.

An owner who grasps that one point has the argument that matters and does not need the molecular detail. It is also the point that settles most disagreements about whether a particular dog needs testing.

2. Merle Is a Spectrum, Not a Switch

2.1 Length Determines Effect

The inserted sequence carries a poly-A tail whose length varies between individuals, and that length co-determines how strongly merle expresses. Across merle breeds, a length spectrum from roughly 200 to 280 base pairs has been described and divided into several categories, with the authors explicitly describing a graded risk of health impairment (Langevin, Synkova, Jancuskova & Pekova, 2018).

2.2 The Variant Categories

Working from the shortest upward: the wild-type allele carries no insertion and produces no merle. Cryptic merle alleles, in the low-200s, produce no visible pattern at all. Cryptic-plus alleles, running into the mid-240s, likewise produce a solid-looking coat. Atypical merle, in the high 240s to low 250s, produces no pattern but sometimes a lightened or brownish tone. Atypical-plus, into the low 260s, can produce weak patterning and a muted, ill-defined color. Classic merle sits in the mid-to-high 260s and produces the recognizable pattern. Harlequin merle, from around 269 upward, produces highly variable patterns including extensive dilution toward white.

Two cautions belong with that list. The boundaries are practical classifications used by testing laboratories rather than sharp biological thresholds, and the transitions between categories are gradual. The allele set, the naming and the overall length range come from the published work; the exact cut-off values are laboratory conventions.

2.3 The Consequence

A dog can carry merle without looking like a merle. Cryptic and atypical variants produce little or no visible pattern, they are inherited normally, and in the following generation they can meet a long allele. The mostly white deaf puppy from two apparently non-merle parents is not a mystery — it is this mechanism working as expected.

2.4 What the Spectrum Means for the Word

Merle as ordinarily used names a look. Merle as the genetics describes it names a range of insertion lengths with different consequences, only some of which are visible on the dog carrying them.

Two people using the word can therefore be discussing different things, which is the source of much of the disagreement in breed communities (with breed explaining less than is usually assumed). One is describing what a dog looks like and the other what it can pass on.

3. Mosaicism: Why Appearance Proves Nothing

3.1 The Finding

The poly-A tail is a highly repetitive sequence and therefore prone to replication error during cell division. The result is that a single dog can carry different merle alleles in different body cells. In the published sample, this genetic mosaicism was found in 30 of 181 dogs — 16.6 percent (Langevin et al., 2018).

3.2 Why It Matters for Breeding

In a mosaic animal, different cell lines carry different merle variants, and which variant is transmitted depends on which cell line gives rise to the germ cells. In males in particular, examining sperm can become relevant, because germline variants may differ from a blood test result — which explains litters that otherwise appear inexplicable.

The authors' conclusion was procedural: mosaic findings must be reported in full to the breeder rather than simplified away.

3.3 What Follows for Owners and Buyers

A breeding partner that "does not look merle" is not evidence of anything. Genetic testing is currently the most reliable way to identify merle variants, though results should still be interpreted in the context of the breeding line.

3.4 Why This Is the Most Consequential Finding in the Article

Everything else could be managed by looking at dogs. Mosaicism removes that option: the visible phenotype is an unreliable guide to what a dog carries and can pass on.

It is also the point most often missing from popular accounts, which still treat merle as something you can see. A breeding program run on appearance alone is running on an unreliable measurement.

4. How the Testing Works

4.1 What a Merle Test Reports

A modern test does not report merle as present or absent. It reports the length of the insertion, which is what determines the phenotype, and a dog can carry two alleles of different lengths, each reported separately (Langevin et al., 2018).

A result reading only merle or non-merle is an older or cheaper test and answers a different question from the one a breeder needs. It will also miss the short variants entirely, which is precisely where the surprises come from.

4.2 Why Length Is the Whole Point

Short insertions produce little or no visible effect; longer ones produce the classic pattern; the longest are associated with the health risks, and the boundaries between these categories are conventions rather than sharp divisions. A dog that looks solid can carry a variant that becomes consequential when paired with another, which is the practical case for testing and the reason this article exists.

4.3 What Mosaicism Does to the Result

Because the insertion is unstable and can shorten during cell division, a dog can carry different lengths in different tissues, and the pattern of that variation is not predictable from the outside. That means a test result reflects the sample taken, and a second sample can read differently.

This is the technical reason appearance proves nothing, and it also limits how definitive any single test result can be. A test is far better than looking at the dog and is not an absolute answer.

4.4 Sampling and Laboratory Choice

Cheek swabs are standard, non-invasive and adequate for most purposes. Where a result is unexpected or breeding decisions depend on it, repeating the test, ideally with a different sample type, is a reasonable precaution given the instability. How often repeat testing changes a result has not been quantified.

Not every laboratory reports length. That is the first thing to establish before paying for a test, and it is a question a laboratory will answer directly if asked.

4.5 What Testing Cannot Tell You

It cannot predict whether a given puppy will be deaf or have ocular defects, because the relationship between genotype and outcome is probabilistic. What it does is identify which matings carry elevated risk (where canine hearing and deafness are treated in detail).

That is a population-level tool used to avoid a category of mating, not an individual prognosis. Breeders who expect the second from it are disappointed, and sellers who present it as the second are overselling.

5. Deafness: The Measured Rates

5.1 The Study

The most robust data come from an objective hearing assessment of 153 merle dogs across breeds, with genotype determined in parallel (Strain, Clark, Wahl, Turner & Murphy, 2009).

Among single merle dogs, 2.7 percent were unilaterally deaf and 0.9 percent bilaterally deaf — around 3.5 percent affected in total. Among double merles, 10 percent were unilaterally deaf and 15 percent bilaterally deaf, for roughly 25 percent affected. The association between hearing status and genotype was statistically significant. There was no significant association with eye color or with sex.

5.2 What This Means for Single Merles

The common claim that single merle carries no health consequence is a simplification. The risk is small but not zero. The overwhelming majority of single merle dogs hear and see normally, which makes a hearing test in puppies worthwhile precisely because unilateral deafness is easily missed in daily life for years.

5.3 Calibration Against Other Breeds

Deafness prevalence in merle dogs exceeded that in some breeds homozygous for the piebald gene, such as the English Cocker Spaniel, but was comparable to or lower than that in Dalmatians and white Bull Terriers (Strain et al., 2009). Merle is one instance of a broader pattern in which reduced pigmentation in sensory tissue carries auditory risk — not a uniquely dangerous gene.

5.4 How to Read the Prevalence Figures

They describe the dogs in that dataset. Applying them to a particular litter assumes the same breed mix, the same variant distribution and the same ascertainment, none of which is guaranteed. A breeder quoting a percentage to a buyer is quoting someone else's population.

What the figures establish reliably is the direction and the rough magnitude, which is enough to inform a breeding decision and not enough to quote as a probability for one puppy.

5.5 Why Unilateral Cases Matter Here

A dog hearing on one side functions well enough that households frequently do not notice, and it still cannot localize sound. For a merle litter that is the commonest finding and the one observation will miss. A puppy that responds to sound has not been shown to hear on both sides.

6. Ocular Effects

6.1 The Clinical Picture

A classical case series described 45 Australian Shepherds with microphthalmia and multiple colobomas. Among affected dogs, microphthalmia, microcornea, and heterochromia with dyscoria and corectopia were each present in all cases; cataracts were found in 62 percent of affected eyes, equatorial staphylomas in 54 percent, and retinal detachments in 53 percent. Affected dogs were 30 to 90 percent white with the remainder blue merle, and cardiac anomalies were noted in six dogs (Gelatt & McGill, 1973).

Read this correctly: these are proportions within already-affected dogs, not prevalence figures for merle dogs generally. There is no ocular equivalent of the Strain hearing dataset.

6.2 Inheritance

Follow-up work characterized microphthalmia with coloboma in the merle Australian Shepherd as an incompletely penetrant recessive trait (Gelatt, Powell & Huston, 1981). Incomplete penetrance means that carrying the relevant genotype does not guarantee the phenotype, which is part of why outcomes in double merle litters vary so widely.

6.3 Pigment and Severity

The most severe ocular findings occur in homozygous merles with extensive white on the head. Dogs with substantial unpigmented skin also lack the protection pigment normally provides, which makes sun exposure a practical management issue rather than a cosmetic one. A dog that avoids bright conditions may be responding to discomfort rather than to the situation (a reading error that recurs across behavior problems).

6.4 What the Ocular Findings Actually Cover

Microphthalmia with coloboma and associated defects, described clinically in a small historical literature. The range of severity is wide, from incidental findings visible only on examination to functional blindness (where conformation-related welfare is treated separately).

Most merle dogs have none of it, which is the context in which the figures should be read. The risk concentrates in double merles, which is why the mating rather than the color is what the recommendations address.

7. Breeding, Law and Testing

7.1 The German Legal Frame

Section 11b of the German Animal Welfare Act prohibits breeding vertebrates where it is to be expected that offspring will hereditarily lack body parts or organs, or have them unsuited or altered, in ways that cause pain, suffering or damage.

Applied to merle, the deliberate pairing of two merle carriers is the constellation in which impaired offspring are to be expected. This is a description of the statutory test, not legal advice; specific breeding questions belong with the responsible veterinary authority.

7.2 What Reduces Risk

Merle to non-merle substantially reduces the risk of a double merle outcome — but only if the genetic variants of both parents are actually known. Given cryptic and atypical alleles, "looks non-merle" does not establish "is non-merle," which is why testing rather than inspection is the operative safeguard.

Professional bodies additionally recommend testing dogs with a merle parent or merle siblings before breeding regardless of breed, testing puppies from merle pairings at the M locus before rehoming, and informing prospective owners of intact merle dogs about the risks in writing.

7.3 Recent Introductions Are the Higher-Risk Case

Merle is long-established in a number of breeds, including the Australian Shepherd, Border Collie, Collie in both coat types, Shetland Sheepdog, Cardigan Welsh Corgi, Dachshund, Great Dane in connection with harlequin, and the Catahoula Leopard Dog. In others — French Bulldog, Chihuahua, Pomeranian, and various bully types — it appears through recent outcrossing driven by market demand.

The recent introductions are treated as the more critical situation, because the gene's effects differ between breeds and newly introduced lines have no accumulated experience to draw on. No complete list of merle-carrying breeds exists, and through mixed breeding merle can appear anywhere.

7.4 Why Recent Introductions Carry More Risk

Where merle has been introduced into a breed that did not previously carry it, the variants present are less well characterized, the population has less testing history, and the commercial pressure to produce the color is higher.

That combination is the one to be most careful about, and it describes several currently fashionable situations, where a color new to a breed commands a premium and the incentives run against careful testing.

8. What to Ask a Breeder

8.1 Were Both Parents Tested, and for Length

Not whether they are merle, but what the test reported. A breeder who can answer that has engaged with the genetics; one who answers by describing the coat has not. The question is also a fair one, and a good breeder will not take offence at it.

8.2 What the Mating Was Expected to Produce

A merle-to-merle mating is the one to avoid, and with length testing it is avoidable in advance. Where such a mating happened anyway, the question is why, and the answer is sometimes that nobody knew one of the parents carried a variant.

Hidden merles are the reason this matters: a dog that looks solid can carry a variant, which is precisely what testing exists to catch. Cryptic merle and phantom merle are the terms most often used for this, and both describe the same situation.

8.3 Have the Puppies Been BAER Tested

Hearing cannot be assessed reliably by observation, and unilateral deafness in particular is routinely missed in a litter. Where merle is involved, testing before homing is the responsible standard, and it costs a fraction of what the puppy does.

8.4 Has an Ophthalmologist Seen Them

Ocular defects associated with merle are identifiable on examination and not from the outside, which means an unexamined puppy has not been shown to be clear. A veterinary ophthalmologist's report is a different document from a general health check, and a breeder who has arranged one has taken a step most do not.

8.5 What to Do If the Answers Are Vague

Vagueness about testing is information. It does not necessarily indicate a bad breeder, and it does indicate that the buyer will be finding these things out themselves after the puppy arrives, at their own cost and on their own timetable.

A puppy with a sensory impairment is a workable proposition for a prepared household and a considerably harder one for a surprised household (where the sensitive period is examined at length). Most of the difficulty lies in the surprise rather than in the impairment.

9. Living and Training with Sensory Impairment

9.1 Most Merle Dogs Need Nothing Special

A single merle with intact hearing and normal vision is an ordinary dog. What is worth doing anyway: a hearing test in puppies from merle pairings, annual eye and ear checks, and sun protection where skin is unpigmented. Early detection matters because a puppy with undiagnosed unilateral deafness moves through its socialization window with incomplete information.

One point is easy to overlook. A dog with reduced perception may register or report discomfort differently, which makes behavioral change worth taking seriously (since pain often presents behaviorally).

9.2 Learning Is Unchanged; the Channel Changes

A dog that cannot see or hear well learns exactly as any other dog does. Learning principles are unaffected — what changes is the channel through which information arrives (the underlying mechanisms being identical).

The governing principle is to announce rather than surprise. Reduced warning time is what produces startle responses, withdrawal or defensive behavior in these dogs — not temperament (which is a state effect rather than a trait one). Repeated unannounced contact is also the reliable way to build a conditioned response to being touched (by exactly the mechanism fear learning describes).

9.3 Working Channels

Scent is the strongest remaining channel and the most rewarding to work with (given how central olfaction is). Consistent scent markers at fixed points — sleeping place, water bowl, the door to outside — build a navigable map.

Touch functions as a full signal system when executed consistently: a flat hand on the chest for stop, a touch at the croup for move on, a tap at the shoulder for a direction change. What matters is that each signal always arrives at the same place, with the same intensity, meaning the same thing — and that it is introduced while the dog is calm enough to process it (which the arousal literature makes a precondition). Build them as any other behavior — shape first, consolidate, then add the tactile cue — rather than the reverse.

Abrupt leash pressure is explicitly not a tactile cue. A signal has to be discriminable, not unpleasant, and a dog with less warning than usual is the last one that should receive an input it cannot see coming (which is where aversive input does its damage).

9.4 Environment and Social Contact

Keep furniture and routes stable, maintain predictable feeding and walking times (since unpredictability is itself a stressor), and treat off-lead freedom as a genuine risk — a long line on a well-fitted Y-harness preserves movement without losing control (predictability being a documented buffer against anxiety).

A dog that cannot fully read other dogs' signals benefits from calm, confident partners and suffers under pushy ones (since signal reading is itself learned). Quality clearly beats quantity here, and a stable relationship with the handler carries more weight than usual (as the attachment literature indicates).

9.5 What Changes and What Does Not

Learning does not change. A deaf dog acquires behavior by the same processes as any other and the marker arrives visually rather than audibly, with the same timing requirements.

What changes is management: recall in open space, waking a sleeping dog, and the fact that the dog cannot hear a car it cannot see. Long lines, vibration collars used as attention cues rather than as corrections, and deliberate practice at being approached while asleep cover most of it.

9.6 The Individual Matters More Than the Category

A deaf dog's temperament, confidence and history shape its life far more than the deafness does, and treating the impairment as the defining fact misdescribes most of these animals, with households frequently reporting that it stopped being a consideration within months.

10. Where the Evidence Comes From

10.1 A Small and Uneven Literature

Five sources carry this article, and they are not of equal age or design. That is worth stating because the topic generates strong opinions on a thin base, and because the thinness is not obvious from how confidently the figures circulate.

10.2 The Genetics Is Recent and Solid

The identification of the retrotransposon insertion (Clark et al., 2006) and the characterization of the length variants and their phenotypes (Langevin et al., 2018) are the foundation, and both are specific to dogs.

The Langevin work in particular changed what the word merle means, from a category to a spectrum, and much popular material has not caught up. Anything written before 2018 describes a simpler picture than the one now established.

10.3 The Deafness Data Are a Single Large Study

The prevalence figures come from one dataset (Strain et al., 2009). It is a good study and it is one study, with the sample composition and breed mix that implies. Independent replication in other populations would settle how far the figures travel.

10.4 The Ocular Work Is Over Forty Years Old

The clinical description of microphthalmia with coloboma and the inheritance work date from the 1970s and early 1980s (Gelatt & McGill, 1973; Gelatt et al., 1981). They remain the reference because nothing has replaced them, which is a statement about the field rather than about their quality.

Ophthalmic imaging, genetic characterization and study design have all moved considerably since. A modern re-examination would be straightforward and has not been done. The dogs exist, the imaging exists, and the genetic characterization now exists too.

10.5 What a Reader Should Take From That

The genetic account is current and reliable. The risk figures rest on a narrow base, and the ocular material is historical. None of that makes the recommendations wrong; it makes the numbers less precise than they appear when quoted. The recommendations follow from the direction of the findings rather than from their exact values.

10.6 What This Article Is Arguing

Not that merle dogs are unhealthy, which most are not, and not that the color should be avoided. The argument is narrower: that appearance is an unreliable guide to genotype, that one category of mating carries clearly elevated risk, and that both are avoidable with a test that exists.

Everything contentious about this topic sits around that core rather than in it, which is worth remembering when the argument becomes heated.

10.7 Why the Debate Runs Hot

Color is commercially valuable, breeding decisions are personal, and the genetics is recent enough that many people learned an older and simpler version of it. Those three together produce disagreements in which the participants are often not discussing the same thing.

Agreeing what merle means genetically resolves a surprising proportion of them.

10.8 The Practical Summary

Test for length rather than mere presence. Avoid merle-to-merle matings. BAER test litters where merle is involved. Have an ophthalmologist examine them. And treat a dog with a sensory impairment as a dog first.

10.9 Why Double Merle Is the Term That Matters

Almost every serious welfare problem discussed here arises in dogs carrying two consequential variants. The single merle is the ordinary case and, on the available evidence, generally the unremarkable one.

Collapsing the two into one conversation about merle dogs is what makes the topic confusing, and separating them is what makes the recommendations tractable. One category of mating, avoidable in advance, accounts for most of the harm.

10.10 Why the Color Itself Is Not the Problem

Nothing about the coat pattern itself harms a dog. The mutation that produces it also affects other tissues where pigment cells have a developmental role, which is a different statement and the one the evidence supports.

That distinction matters because arguments framed against the color lose people who own perfectly healthy merle dogs and can see for themselves that the claim is overstated. An accurate argument persuades where an exaggerated one does not.

10.11 What a Buyer Actually Controls

Not the genetics, which were settled before the puppy existed. What a buyer controls is which breeder they go to and what they ask, and both of those are decided before any money changes hands.

After that point the questions in this article are still worth asking and answer a different purpose: preparing for what is there rather than choosing what will be.

11. Summary at a Glance

Merle is a retrotransposon insertion, not a point mutation — A SINE of roughly 253 base pairs at the intron 10 / exon 11 boundary of PMEL on chromosome 10 (Clark et al., 2006).

Incompletely dominant — One copy gives the pattern; two give a qualitatively different and higher-risk phenotype, which is why merle-to-merle is the problem case.

A spectrum, not a switch — Poly-A tail length from roughly 200 to 280 base pairs sorts into cryptic, atypical, classic and harlequin variants with graded risk (Langevin et al., 2018).

Carriers can look unremarkable — Cryptic and atypical alleles produce little or no visible pattern and are inherited normally.

Mosaicism was found in 16.6 percent — 30 of 181 dogs carried different merle alleles in different cells, so a blood test may differ from what is transmitted (Langevin et al., 2018).

Deafness: 3.5 percent versus 25 percent — Single merles 2.7 percent unilateral and 0.9 percent bilateral; double merles 10 percent unilateral and 15 percent bilateral, across 153 dogs (Strain et al., 2009).

No eye-color or sex association — Neither predicted hearing status (Strain et al., 2009).

Ocular findings are severe but not quantified as prevalence — The classical series describes what affected dogs have (Gelatt & McGill, 1973), not how many merle dogs are affected.

12. Research Gaps and Critical Appraisal

There is no ocular equivalent of the deafness dataset. The Strain study gives genotype-linked prevalence for hearing. Nothing comparable exists for eyes: the ocular literature is built on case series of affected animals, which characterizes severity well and prevalence not at all.

The allele cut-offs are laboratory conventions. The variant set, the naming and the overall length range are published. The precise base-pair boundaries between categories are practical classifications used by testing laboratories, and the biological transitions are gradual rather than stepped.

Breed-specific risk is largely unquantified. The deafness sample spanned multiple breeds, and effects are known to differ between breeds — but breed-level figures are unavailable for most, which is precisely the situation in recently outcrossed lines where risk assessment matters most.

The core ocular work is over four decades old. Gelatt and McGill's series dates from 1973 and the inheritance study from 1981, both predating molecular characterization of the locus. They have not been superseded by comparable modern work.

Mosaicism has been described but not systematically quantified across breeds. A single study established the phenomenon and its frequency in one sample. How it varies between populations, and how often germline and somatic results diverge in practice, remains open.

Welfare outcomes for affected dogs are not systematically tracked. How double merle dogs with sensory impairment actually fare over a lifetime — behaviorally, medically, in terms of rehoming success — is not documented in any structured way (and behavioral outcomes are hard to read directly).

The ocular literature is historical. The clinical and inheritance work dates from 1973 to 1981 (Gelatt & McGill, 1973; Gelatt et al., 1981) and has not been re-examined with modern imaging or genetic characterization.

The deafness figures rest on one dataset. Prevalence comes from a single large study (Strain et al., 2009), with the breed mix and ascertainment that implies.

Mosaicism limits any single test result. Because the insertion is unstable, a result reflects the tissue sampled, and how often that matters in practice has not been quantified.

No outcome data follow merle dogs over time. How affected animals fare across a lifetime, and what predicts good outcomes, has not been studied.

13. Conclusion

Merle is neither harmless nor catastrophic; it is a breeding decision with quantified consequences, and the numbers are specific enough to argue from. A single merle dog carries a small but non-zero risk of deafness at roughly 3.5 percent, which makes the common reassurance that single merle is entirely safe a simplification rather than a fact. A double merle carries around 25 percent, with 15 percent bilaterally deaf, alongside ocular malformations that can be severe. What turns this from a genetics curiosity into a practical welfare problem is that appearance does not identify carriers: cryptic and atypical variants produce dogs that look entirely ordinary and transmit merle normally, and genetic mosaicism means even a test result reflects the cells sampled rather than necessarily the cells that will produce offspring. The operational conclusions follow directly. Test rather than inspect, ask for both parents' genotypes before buying, and treat a refusal to produce them as the answer it is. For the dogs already living with impairment, the picture is more ordinary than it sounds — learning principles are unchanged, only the channel differs, and a dog navigating by scent and touch on predictable routes is not a diminished dog but one working with a different set of inputs (much as counterconditioning works through whichever channel is available).

Key Insights (Takeaways)

• A dog can carry merle without looking like one. Cryptic and atypical variants of the insertion produce little or no visible pattern while being inherited normally (Langevin et al., 2018), which is how two apparently non-merle parents produce an affected puppy. Inspection is not a substitute for a genetic test, and this single fact drives most of the practical guidance in this area (the MDR1 defect and drug sensitivity).

• The deafness difference between genotypes is large and measured. Across 153 dogs, 3.5 percent of single merles were deaf in at least one ear, against 25 percent of double merles, with 15 percent of double merles deaf bilaterally (Strain et al., 2009). Eye color and sex predicted nothing. The claim that single merle carries no risk is a simplification; the risk is small, not absent.

• Merle is incompletely dominant, and that is the whole argument. Two copies do not produce more pattern but a qualitatively different phenotype with substantially higher risk — which is why merle-to-merle pairing, rather than merle itself, is the welfare issue.

• Genetic mosaicism was found in 16.6 percent of a sampled population. Thirty of 181 dogs carried different merle alleles in different cells (Langevin et al., 2018), meaning a blood test may not reflect what is transmitted through the germline — and that mosaic findings must be reported to breeders in full rather than simplified.

• The ocular evidence describes severity, not prevalence. The classical series documents what affected dogs have — microphthalmia and microcornea in all cases, cataracts in 62 percent of affected eyes, retinal detachment in 53 percent (Gelatt & McGill, 1973) — but no study gives genotype-linked ocular prevalence the way the hearing data does. Quoting these figures as merle risk rates misreads them.

References

Clark, L. A., Wahl, J. M., Rees, C. A., & Murphy, K. E. (2006). Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. Proceedings of the National Academy of Sciences, 103(5), 1376–1381. https://doi.org/10.1073/pnas.0506940103

Gelatt, K. N., & McGill, L. D. (1973). Clinical characteristics of microphthalmia with colobomas of the Australian Shepherd Dog. Journal of the American Veterinary Medical Association, 162(5), 393–396. https://pubmed.ncbi.nlm.nih.gov/4691375/

Gelatt, K. N., Powell, N. G., & Huston, K. (1981). Inheritance of microphthalmia with coloboma in the Australian Shepherd Dog. American Journal of Veterinary Research, 42(10), 1686–1690. https://pubmed.ncbi.nlm.nih.gov/7325429/

Langevin, M., Synkova, H., Jancuskova, T., & Pekova, S. (2018). Merle phenotypes in dogs – SILV SINE insertions from Mc to Mh. PLoS ONE, 13(9), e0198536. https://doi.org/10.1371/journal.pone.0198536

Strain, G. M., Clark, L. A., Wahl, J. M., Turner, A. E., & Murphy, K. E. (2009). Prevalence of deafness in dogs heterozygous or homozygous for the merle allele. Journal of Veterinary Internal Medicine, 23(2), 282–286. https://doi.org/10.1111/j.1939-1676.2008.0257.x