Merle Gentics I
Basic description for the beginner: Merle is not a coat color but a gene that lightens or dilutes the coat. There are several ways to produce a merle. The most straight forward way to produce a merle (Mm) is to breed it to one that is not-a-merle (mm). This breeding will produce both merle (Mm) and non-merle puppies (mm). The second which is not recommended for the novice is the breeding together of two merles (Mm + Mm). Such a breeding could produce what is known as a double merle (MM). The offspring in this case would be made up of 50% merles (Mm), 25% double merles (MM), and 25% non-merles (mm). The final way is to breed a Phantom merle to a non-merle.
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The merle coat color is characterized by patches of dilute pigment in combination with areas of full pigmentation. Therefore, the merle gene acts to lighten whatever coat color would otherwise be expressed. However, unlike other dilution genes, the lightening effect is not spread evenly over the coat, but is expressed as patches of diluted color scattered over the dog’s body. If the basic color of the dog is black, the effect of the merle gene is a soft gray, often referred to as “blue”. If the basic color of the dog is red, the effect of the merle gene is a pale red. The merle coat pattern is characteristic of a number of breeds recognized by the American Kennel Club, including the Shetland Sheepdog, Collie, Border Collie, Dachshund, Australian Shepherd, and Cardigan Welsh Corgi.
The merle gene (M) is inherited in an autosomal fashion. In other words, the trait is not linked to gender and can be passed on from either the mother or the father. The gene is incompletely dominant, or a gene that has intermediate expression. A heterozygous dog, carrying only one copy of the merle gene (Mm), expresses the characteristic diluted coat color pattern. A non-merle dog (mm) is normal in color, while a homozygous double-merle (MM) is predominantly white. Punnett squares can be used to determine the expected coat color of offspring when breeding dogs of known genotype (i.e. coat color genes have been identified). In the example illustrated, a non-merle dog (mm), indicated in the vertical column, bred to a heterozygous merle (Mm), indicated in the horizontal column, will give rise to offspring with an expected frequency of 50% merle (Mm) and 50% non-merle (mm). Dogs that carry the merle gene but do not show the characteristic merle phenotype, are known as cryptic merles. These dogs may give rise to merle offspring. It is suspected that the DNA sequence of the merle allele in the cryptic is shorter than the allele expressed in the typical merle dog. The harlequin coat color pattern in Great Danes is produced through the interaction of the merle locus and the harlequin (H) gene. In harlequin Danes, the merle background color is diluted to nearly white with fully pigmented black patches.
Both heterozygous merle (Mm) and homozygous double merle (MM) dogs may exhibit auditory and ophthalmic abnormalities including mild to severe deafness, increased intraocular pressure, ametropia, microphthalmia and colobomas. The double merle genotype may also be associated with abnormalities of skeletal, cardiac and reproductive systems.
Tri-Colored non-merle Shetland Sheepdog (mm)
Tri-Colored merle Shetland Sheepdog (Mm)
Tri-Colored double-merle Shetland Sheepdog (MM)
ResourceMaterial: GenMark DNA Technology Service of Vita Tech Labortories LLC
*For more info. on double merles, go to outside link Double Merles (MM)
Homepage address: www.whiteaussies.com
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The merle coat color is characterized by patches of dilute pigment in combination with areas of full pigmentation. Therefore, the merle gene acts to lighten whatever coat color would otherwise be expressed. However, unlike other dilution genes, the lightening effect is not spread evenly over the coat, but is expressed as patches of diluted color scattered over the dog’s body. If the basic color of the dog is black, the effect of the merle gene is a soft gray, often referred to as “blue”. If the basic color of the dog is red, the effect of the merle gene is a pale red. The merle coat pattern is characteristic of a number of breeds recognized by the American Kennel Club, including the Shetland Sheepdog, Collie, Border Collie, Dachshund, Australian Shepherd, and Cardigan Welsh Corgi.
Genetic Inheritance of the Merle Gene
The merle gene (M) is inherited in an autosomal fashion. In other words, the trait is not linked to gender and can be passed on from either the mother or the father. The gene is incompletely dominant, or a gene that has intermediate expression. A heterozygous dog, carrying only one copy of the merle gene (Mm), expresses the characteristic diluted coat color pattern. A non-merle dog (mm) is normal in color, while a homozygous double-merle (MM) is predominantly white. Punnett squares can be used to determine the expected coat color of offspring when breeding dogs of known genotype (i.e. coat color genes have been identified). In the example illustrated, a non-merle dog (mm), indicated in the vertical column, bred to a heterozygous merle (Mm), indicated in the horizontal column, will give rise to offspring with an expected frequency of 50% merle (Mm) and 50% non-merle (mm). Dogs that carry the merle gene but do not show the characteristic merle phenotype, are known as cryptic merles. These dogs may give rise to merle offspring. It is suspected that the DNA sequence of the merle allele in the cryptic is shorter than the allele expressed in the typical merle dog. The harlequin coat color pattern in Great Danes is produced through the interaction of the merle locus and the harlequin (H) gene. In harlequin Danes, the merle background color is diluted to nearly white with fully pigmented black patches.
Health Problems Associated with the Merle Allele
Both heterozygous merle (Mm) and homozygous double merle (MM) dogs may exhibit auditory and ophthalmic abnormalities including mild to severe deafness, increased intraocular pressure, ametropia, microphthalmia and colobomas. The double merle genotype may also be associated with abnormalities of skeletal, cardiac and reproductive systems.
Tri-Colored non-merle Shetland Sheepdog (mm) Tri-Colored merle Shetland Sheepdog (Mm) Tri-Colored double-merle Shetland Sheepdog (MM)ResourceMaterial: GenMark DNA Technology Service of Vita Tech Labortories LLC
*For more info. on double merles, go to outside link Double Merles (MM)
Homepage address: www.whiteaussies.com
Merle Genetics II
Merle is a dilution gene, that is, it lightens whatever the coat color would otherwise have been. The lightening is not spread evenly over the coat, but leaves patches of undiluted color scattered over the dog's body. Also, the lightening seems to work primarily on the black pigment in the coat, so any tan on the face stays even. Note that "black" as used here includes liver or chocolate. These colors are rare color faults in Shelties, but everything written here applies also to other breeds with the merle gene, including Australian Shepherds. A red merle in that breed is produced by the merle gene acting on a liver (solid red-brown, not the same as sable) coat.
One dose of the merle gene on an otherwise black dog produces a blue merle - a more or less bluish gray dog dappled with black spots. Tan points - the tan spots over the eyes, on the sides of the muzzle, on the legs and under the tail of a tricolor dog - will still be there in the merled tricolor. If the tan spots would not be present in a black dog, giving what is usually called a bi-black in Shelties, tan will not be present in the merled black either, and the dog will blue merle and white without tan: a bi-blue. One dose of the merle gene on an otherwise sable dog produces a sable merle. Sable merles are less predictable in color than blue merles, and may range anywhere from an apparent sable, often with a pinkish or orange cast to its coat, to something that looks like a very rusty blue merle. White markings remain on the merled dog, and may even be slightly more prominant.
Merle acts on the black pigment in the iris of the eye just as it does on the coat, so merle dogs often have part or all of the eye blue. (This does not affect their vision, though since it happens to some extent in the retina as well it may make it harder to diagnose certain eye problems.) The Sheltie breed standard allows blue or merle eyes in blue merles, but not in sables. Thus a sable merle with blue or merle eyes will not do well in the show ring. Notice that I said a single dose of the merle gene. There are always two copies of a gene, alike or different, in any dog. If we call the merle gene M and the non-merle gene m, any given dog can be mm, Mm or MM. The mm dog is the normal, full-colored tri, bi-black, or sable in Shelties, or liver (red) in Australian Shepherds. The Mm Sheltie is a blue merle or sable merle, depending on what color it would have been without the merling gene. An MM dog, often called a double merle or a homozygous merle, will be mostly white and usually deaf or blind and often with other physical problems. Some MM puppies are born completely without eyes.
On average over a large number of litters, breeding merle to merle will produce one fourth full colored dogs, one half merles and one fourth defective whites. Breeding merle to full color will produce one half full color and one half merles, but no defective whites. The merle to full color breeding, then, produces just as many merles as does the merle to merle breeding, and without the danger of defective puppies. The safe breeding for a merle, then, is to a non-merle mate. This breeding should produce all healthy puppies, and about half will be merles.
To breed in this way, it is important to know which dogs are merles. This is one of the reasons experienced breeders rarely breed blue merles to sables, as this mating may produce sable merles. Sable merles are no more likely to have health problems than any other color, and they are equally good companions. Many do have colors that are not accepted in the show ring, either because they have blue or merle eyes or because the mottling produced by the merle gene is too obvious. The real arguement against sable merles is that they may be mistaken for normal sables. If two such sable merles were mated together, the resulting litter could contain defective whites. What a shock for the breeder expecting normal, healthy puppies! There is one kind of breeding that can produce all or almost all merles, and that is the breeding of a tricolor or a bi-black to a double merle - but remember that the double merle has a high probability of being blind or deaf. A very few breeders have been lucky enough to get high quality homozygous merles that are not too severely affected to breed - but it definitely takes a lot of luck and really top quality blue merles to start with. Merle to merle breedings are only for the very experienced breeder who knows her lines and what they will produce - and it has probably produced more heartbreaks than good homozygous merles, even for them. A blue merle from black to homozygous merle breeding is just as healthy as one from a more normal black to blue merle breeding.
Unless you have done a lot of merle breeding and really know what you are getting into, the safe rule is still that a blue merle should be bred only to a black (tri or bi). Note that in Shelties, all blue merles imported to this country can be traced in direct merle-to-merle line to crosses involving blue merle Collies.
One dose of the merle gene on an otherwise black dog produces a blue merle - a more or less bluish gray dog dappled with black spots. Tan points - the tan spots over the eyes, on the sides of the muzzle, on the legs and under the tail of a tricolor dog - will still be there in the merled tricolor. If the tan spots would not be present in a black dog, giving what is usually called a bi-black in Shelties, tan will not be present in the merled black either, and the dog will blue merle and white without tan: a bi-blue. One dose of the merle gene on an otherwise sable dog produces a sable merle. Sable merles are less predictable in color than blue merles, and may range anywhere from an apparent sable, often with a pinkish or orange cast to its coat, to something that looks like a very rusty blue merle. White markings remain on the merled dog, and may even be slightly more prominant.
Merle acts on the black pigment in the iris of the eye just as it does on the coat, so merle dogs often have part or all of the eye blue. (This does not affect their vision, though since it happens to some extent in the retina as well it may make it harder to diagnose certain eye problems.) The Sheltie breed standard allows blue or merle eyes in blue merles, but not in sables. Thus a sable merle with blue or merle eyes will not do well in the show ring. Notice that I said a single dose of the merle gene. There are always two copies of a gene, alike or different, in any dog. If we call the merle gene M and the non-merle gene m, any given dog can be mm, Mm or MM. The mm dog is the normal, full-colored tri, bi-black, or sable in Shelties, or liver (red) in Australian Shepherds. The Mm Sheltie is a blue merle or sable merle, depending on what color it would have been without the merling gene. An MM dog, often called a double merle or a homozygous merle, will be mostly white and usually deaf or blind and often with other physical problems. Some MM puppies are born completely without eyes.
On average over a large number of litters, breeding merle to merle will produce one fourth full colored dogs, one half merles and one fourth defective whites. Breeding merle to full color will produce one half full color and one half merles, but no defective whites. The merle to full color breeding, then, produces just as many merles as does the merle to merle breeding, and without the danger of defective puppies. The safe breeding for a merle, then, is to a non-merle mate. This breeding should produce all healthy puppies, and about half will be merles.
To breed in this way, it is important to know which dogs are merles. This is one of the reasons experienced breeders rarely breed blue merles to sables, as this mating may produce sable merles. Sable merles are no more likely to have health problems than any other color, and they are equally good companions. Many do have colors that are not accepted in the show ring, either because they have blue or merle eyes or because the mottling produced by the merle gene is too obvious. The real arguement against sable merles is that they may be mistaken for normal sables. If two such sable merles were mated together, the resulting litter could contain defective whites. What a shock for the breeder expecting normal, healthy puppies! There is one kind of breeding that can produce all or almost all merles, and that is the breeding of a tricolor or a bi-black to a double merle - but remember that the double merle has a high probability of being blind or deaf. A very few breeders have been lucky enough to get high quality homozygous merles that are not too severely affected to breed - but it definitely takes a lot of luck and really top quality blue merles to start with. Merle to merle breedings are only for the very experienced breeder who knows her lines and what they will produce - and it has probably produced more heartbreaks than good homozygous merles, even for them. A blue merle from black to homozygous merle breeding is just as healthy as one from a more normal black to blue merle breeding.
Unless you have done a lot of merle breeding and really know what you are getting into, the safe rule is still that a blue merle should be bred only to a black (tri or bi). Note that in Shelties, all blue merles imported to this country can be traced in direct merle-to-merle line to crosses involving blue merle Collies.
Resource: http://bowlingsite.mcf.com/Genetics/Merle.html
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