Colour Genetics
This is just a slight insight into dog colour and genetics there is a lot more to it, and just remember don't just pick a puppy/dog for colour. Remember health, temperament and over all well being of pup comes first and is your lifestyle right for it.
The Dominant Black Gene
Black is the "default" eumelanin colour for dogs. A dog which isn't homozygous for liver (bb) or for dilution (dd) will have black eumelanin. This means that it will have a black nose and, usually, brown eyes (eumelanin affects eye colour too), and any eumelanin in its coat will be black. Eumelanin, in case you missed out the introduction pages, is one of two types of pigment that occur in dogs. The other is phaeomelanin, which doesn't affect the eyes or nose and is only visible in the coat. It produces the colour "red", which is anything from deep Irish Setter red to light cream. Phaeomelanin doesn't "naturally" occur in the coat - it only appears if the dog has genes which allow it to occur.
There are two basic choices for a dog's markings - solid (no tan markings, just eumelanin) or non-solid (tan markings of any sort). Whether a dog has a solid eumelanin (black) coat or a coat with tan markings (caused by phaeomelanin) depends almost entirely on the K locus. K consists of three alleles:
K - dominant black (solid black, no red)
k - recessive non-black (will still have black nose pigment and may have black markings, but may also have red markings too)
kbr - brindle (we won't deal with this here - see the brindle page for more info on this allele)
A dog with even just one K gene will be solid black. A dog with two k genes (i.e. homozygous for k) will be able to show tan markings. These tan markings are determined by another locus, A. So basically, a genotype of kk allows a dog to show whatever it has on the A locus. A Kk or KK dog may be genetically tan-pointed or sable on the A locus, but won't be able to show those markings because of its dominant black genes. Dominant black dominates the whole of the A locus, but it can be overridden by other genes, such as liver, dilution, greying and merle. All of these will alter the way a dominant black dog looks, but the one thing they cannot do is add phaeomelanin (red) to the coat. The only way phaeomelanin can be added to the coat of a dog with the dominant black gene is through the e gene (E locus) - recessive red. This turns a dominant black dog (or indeed, any dog) into a solid red dog with black nose pigment.
This all probably sounds very confusing at this point, and if you have no background in genetics you may not understand everything. Don't worry - we'll deal with the genes that can affect black on different pages a bit later on. For now, all you really need to know is that Kk and KK on the K locus produce a solid black dog. A kk dog may have some black in its coat, but it won't usually be solid black. The stuff about eumelanin and phaeomelanin is particularly confusing, but I've made sure to use those terms all throughout this site because if you can get a handle on them, you'll be able to understand dog genetics easily.
The Liver Gene
The liver gene occurs on the B locus. It is recessive, so b is liver and B is non-liver, and in order for a dog to be liver it must have the genotype bb. This means that a liver puppy can be born from black parents if both are carriers of the liver gene (i.e. if both are Bb then at least one pup in four will be bb
The different pigment colour genotypes are:
BBdd or Bbdd - blue (non-liver, dilute)
BBDd or BBDD - black (non-liver, non-dilute)
bbdd - isabella (liver, dilute)
bbDd or bbDD - liver (liver, non-dilute)
The liver gene affects eumelanin (black pigment) only. All of the black in the coat will be turned to liver when a dog is bb on the B locus. This includes saddles, shading, merle etc. It is genetically impossible for a liver dog to have even one black or even grey hair in its coat, or for a black or blue dog to have liver in its coat (although bronzing and seal may appear liver). The entire coat on a liver will be shades of brown, with red (tan) or white according to the other genes present.
The Dilution Gene
The dilution gene occurs on the D locus. It is recessive, so d is dilute and D is non-dilute, and in order for a dog to be dilute it must have the genotype dd. A dog which is Dd or DD will have normal pigment.
The dilution gene affects eumelanin (black and liver), although phaeomelanin (red) may be lightened slightly as well. When a dog has two copies of the d gene it impairs its ability to make full-coloured pigment, so the pigment it does produce is paler than on a normal dog. A black dog with the dilution gene becomes blue and a liver dog becomes isabella. A blue or isabella can have any coat pattern, but whatever they have, any black or liver in the coat will be turned to blue or isabella. It is genetically impossible for a blue dog to have any black in its coat, or for an isabella to have liver.
The main giveaway that a dog is a dilute is its nose colour. The coat may be entirely sable or recessive red, but if the dog has a blue nose, it is genetically blue.
The Agouti Series
The agouti series consists of:
Ay - sable
aw - agouti
at - tan points
a - recessive black
The Brindle Gene
Brindle is one of the recessive genes on the K locus. There are three K locus genes - K (dominant black), kbr (brindle) and k (non-solid black - allows A locus to be expressed). K is the top dominant and k is the bottom recessive. kbr sits between the two. It is entirely dominated by K (so just one K gene will stop brindle from being expressed), but is dominant over k, so a brindle dog can have the genotype kbrkbr or kbrk.
A dog with one or two kbr genes will express whichever genes it has on the A locus, but any and all phaeomelanin (red) in the coat will be brindled. This means that the extent of the brindling on the coat depends on the A locus. The following list shows how the different A locus genes affect the appearance of brindle.
Brindle + AyAy (sable) - dog will be solid brindle (possibly with darker areas on the back and head where the black tipping on the sable would be)
Brindle + atat (tan points) - dog will be black with brindle points
Brindle + awaw (agouti/wolf grey) - effect on dog is unknown. Would most likely appear to have brindle points, or brindle may not appear at all
Brindle + aa (recessive black) - dog will be solid black (recessive black does not allow the production of phaeomelanin in the coat)
Brindle typically appears as black stripes on a red base. The stripes are eumelanin and the base is phaeomelanin, so the appearance of those pigments can be changed by any of the genes which usually affect them.
- Eumelanin (the pigment making up the stripes) can be affected by: merle (and harlequin), liver, dilution, greying, and recessive red.
- Phaeomelanin (the pigment making up the base) can be affected by: Intensity locus.
And of course, white markings and ticking can occur on any brindle dog.
The Merle Gene
The M locus is the home of the merle allele. Merle is dominant, and so denoted by the capital letter M. Non-merle is recessive, and denoted by m.
Merle is pretty unique because all normal merles are heterozygous (Mm or mM). A homozygous merle is actually a double merle, similar to the lethal white gene found in horses and other mammals (although not itself lethal). Click here to go to the page on double merle.
The merle gene dilutes random sections of the coat to a lighter colour (usually grey in a black-pigmented dog), leaving patches of the original colour remaining. The patches can be any size and can be located anywhere, unlike the patches on a piebald dog (which are generally confined to the body and head). The edges of the patches may appear jagged and torn.
Merle affects only eumelanin. That means that any black, liver, blue or isabella in the coat, eyes or nose will be merled, whether it's the whole of the body, a mask on a sable, shading, brindle stripes, or even a saddle. Phaeomelanin (red) is not affected at all and will appear as normal.
Health Problems Linked to Colour
There are a few colour genes which can occasionally cause health problems in dogs, most notably merle.
Double Merles and High Whites
A homozygous (or "double") merle is one with two copies of the merle gene, and this severely impairs its ability to make pigment, leaving large areas of the dog pigmentless (white). Pigment is actually necessary for certain parts of the body to function correctly, so lack of pigment can cause health problems.
Dogs with large amounts of white caused by the homozygous piebald allele (sp), such as Bull Terriers, Boxers and Dalmatians, can also have some of the same health problems as double merles, particularly deafness (which is a huge problem in Dalmatians).
Lack of pigment in particular parts of the inner ear can cause deafness, which can be unilateral (just one ear) or bilateral (both ears). It is commonly claimed that dogs with white ears are always deaf, but in fact it's been shown that whether or not pigment is visible on the outer ear does not affect whether or not the dog can hear. In other words, a dog may have coloured ears but still be deaf, and a dog with white ears will not necessarily have any problems.
The double merle gene can also cause eye deformities. This is because the location of the eye cells in an embryo happens to be the same place that pigment starts to appear. If there is a problem with the pigment, this can therefore affect the development of the eyes. Problems include irregularly-shaped pupils, subluxated pupils (not positioned in the right place), microphthalmia (abnormally small eyes, usually with impaired vision), and other, less visible abnormalities causing blindness and bad vision.
Lack of pigment anywhere on the dog can make the skin much more sensitive to the sun. This is a particular problem on the nose, as it is so exposed, but any area of pink skin is susceptible to sunburn and skin cancer. The same problem occurs with any animal which has little or no pigment. White cats are probably the most well-known example. Skin cancer rates in white cats are extremely high and a surprising proportion of cats with white ears end up having their eartips amputated to stop the spread of cancer. The main way to prevent sunburn in animals is the same as with humans - apply suncream!
Dilutes
There is a common misconception that dilutes are in some way naturally sickly - this is not in fact the case. The dilution gene does impair the ability of the cells to make pigment, but only in that it causes the pigment that is made to be less intense. As with most recessives, the dilute allele is in some way "faulty", but it is only faulty in its ability to produce full-strength eumelanin.
The ability or inability of the cells to produce full-strength eumelanin does not affect the health of the dog, simply its colour.
That said, the idea of dilutes as unhealthy most likely has its foundations in Colour Dilution Alopecia. This is an apparently genetic disease causing hair loss and skin problems. A dog with this disorder will typically appear "mangy" and have partial hair loss. It is usually reported from blue dogs, particularly Dobermanns, but presumably it affects isabella dogs too (diluted livers). Any colour can carry CDA or be homozygous for it, but only blues and isabellas will have symptoms.
CDA does not occur on all dilutes and its frequency varies between breeds. It is particularly common in Dobermanns, occuring in up to 80% of dilute dogs. Dilutes in other species such as mice are caused by the same gene, and yet CDA is not known in these, implying it is not an unavoidable consequence of dilution. It is thought that CDA may be caused by a specific dilution gene - labelled dl. Just as there are various different b alleles that all cause the liver colour (phenotypically the same, so only distinguishable through genetic testing), it is probable that there are a number of different d alleles as well, and only one of these causes CDA.
What this means is that CDA is most likely caused by a recessive allele but could theoretically be bred out of most lines by careful breeding and genetic testing.
This blue German Pinscher appears to have mild alopecia. Its coat is dull rather than having a healthy shine, and it seems thin and patchy.
The same problem can also occur (albeit rarely) on black or liver dogs, and is known as Black Hair Follicular Dysplasia. It affects black/liver hairs only, leaving all other hairs as normal. Because this condition is so rare, it often goes undiagnosed. I used to know a Jack Russell Terrier mix who was white except for a black patch on his back, which was hairless. His condition puzzled a whole string of vets and skin specialists, who suggested various types of mange and allergies, and he was never properly diagnosed as having Black Hair Follicular Dysplasia. Unfortunately for dogs with genetic hair loss conditions, there is no cure, although these conditions do not generally cause the dog to be itchy or uncomfortable and so are mostly harmless.
Black is the "default" eumelanin colour for dogs. A dog which isn't homozygous for liver (bb) or for dilution (dd) will have black eumelanin. This means that it will have a black nose and, usually, brown eyes (eumelanin affects eye colour too), and any eumelanin in its coat will be black. Eumelanin, in case you missed out the introduction pages, is one of two types of pigment that occur in dogs. The other is phaeomelanin, which doesn't affect the eyes or nose and is only visible in the coat. It produces the colour "red", which is anything from deep Irish Setter red to light cream. Phaeomelanin doesn't "naturally" occur in the coat - it only appears if the dog has genes which allow it to occur.
There are two basic choices for a dog's markings - solid (no tan markings, just eumelanin) or non-solid (tan markings of any sort). Whether a dog has a solid eumelanin (black) coat or a coat with tan markings (caused by phaeomelanin) depends almost entirely on the K locus. K consists of three alleles:
K - dominant black (solid black, no red)
k - recessive non-black (will still have black nose pigment and may have black markings, but may also have red markings too)
kbr - brindle (we won't deal with this here - see the brindle page for more info on this allele)
A dog with even just one K gene will be solid black. A dog with two k genes (i.e. homozygous for k) will be able to show tan markings. These tan markings are determined by another locus, A. So basically, a genotype of kk allows a dog to show whatever it has on the A locus. A Kk or KK dog may be genetically tan-pointed or sable on the A locus, but won't be able to show those markings because of its dominant black genes. Dominant black dominates the whole of the A locus, but it can be overridden by other genes, such as liver, dilution, greying and merle. All of these will alter the way a dominant black dog looks, but the one thing they cannot do is add phaeomelanin (red) to the coat. The only way phaeomelanin can be added to the coat of a dog with the dominant black gene is through the e gene (E locus) - recessive red. This turns a dominant black dog (or indeed, any dog) into a solid red dog with black nose pigment.
This all probably sounds very confusing at this point, and if you have no background in genetics you may not understand everything. Don't worry - we'll deal with the genes that can affect black on different pages a bit later on. For now, all you really need to know is that Kk and KK on the K locus produce a solid black dog. A kk dog may have some black in its coat, but it won't usually be solid black. The stuff about eumelanin and phaeomelanin is particularly confusing, but I've made sure to use those terms all throughout this site because if you can get a handle on them, you'll be able to understand dog genetics easily.
The Liver Gene
The liver gene occurs on the B locus. It is recessive, so b is liver and B is non-liver, and in order for a dog to be liver it must have the genotype bb. This means that a liver puppy can be born from black parents if both are carriers of the liver gene (i.e. if both are Bb then at least one pup in four will be bb
The different pigment colour genotypes are:
BBdd or Bbdd - blue (non-liver, dilute)
BBDd or BBDD - black (non-liver, non-dilute)
bbdd - isabella (liver, dilute)
bbDd or bbDD - liver (liver, non-dilute)
The liver gene affects eumelanin (black pigment) only. All of the black in the coat will be turned to liver when a dog is bb on the B locus. This includes saddles, shading, merle etc. It is genetically impossible for a liver dog to have even one black or even grey hair in its coat, or for a black or blue dog to have liver in its coat (although bronzing and seal may appear liver). The entire coat on a liver will be shades of brown, with red (tan) or white according to the other genes present.
The Dilution Gene
The dilution gene occurs on the D locus. It is recessive, so d is dilute and D is non-dilute, and in order for a dog to be dilute it must have the genotype dd. A dog which is Dd or DD will have normal pigment.
The dilution gene affects eumelanin (black and liver), although phaeomelanin (red) may be lightened slightly as well. When a dog has two copies of the d gene it impairs its ability to make full-coloured pigment, so the pigment it does produce is paler than on a normal dog. A black dog with the dilution gene becomes blue and a liver dog becomes isabella. A blue or isabella can have any coat pattern, but whatever they have, any black or liver in the coat will be turned to blue or isabella. It is genetically impossible for a blue dog to have any black in its coat, or for an isabella to have liver.
The main giveaway that a dog is a dilute is its nose colour. The coat may be entirely sable or recessive red, but if the dog has a blue nose, it is genetically blue.
The Agouti Series
The agouti series consists of:
Ay - sable
aw - agouti
at - tan points
a - recessive black
The Brindle Gene
Brindle is one of the recessive genes on the K locus. There are three K locus genes - K (dominant black), kbr (brindle) and k (non-solid black - allows A locus to be expressed). K is the top dominant and k is the bottom recessive. kbr sits between the two. It is entirely dominated by K (so just one K gene will stop brindle from being expressed), but is dominant over k, so a brindle dog can have the genotype kbrkbr or kbrk.
A dog with one or two kbr genes will express whichever genes it has on the A locus, but any and all phaeomelanin (red) in the coat will be brindled. This means that the extent of the brindling on the coat depends on the A locus. The following list shows how the different A locus genes affect the appearance of brindle.
Brindle + AyAy (sable) - dog will be solid brindle (possibly with darker areas on the back and head where the black tipping on the sable would be)
Brindle + atat (tan points) - dog will be black with brindle points
Brindle + awaw (agouti/wolf grey) - effect on dog is unknown. Would most likely appear to have brindle points, or brindle may not appear at all
Brindle + aa (recessive black) - dog will be solid black (recessive black does not allow the production of phaeomelanin in the coat)
Brindle typically appears as black stripes on a red base. The stripes are eumelanin and the base is phaeomelanin, so the appearance of those pigments can be changed by any of the genes which usually affect them.
- Eumelanin (the pigment making up the stripes) can be affected by: merle (and harlequin), liver, dilution, greying, and recessive red.
- Phaeomelanin (the pigment making up the base) can be affected by: Intensity locus.
And of course, white markings and ticking can occur on any brindle dog.
The Merle Gene
The M locus is the home of the merle allele. Merle is dominant, and so denoted by the capital letter M. Non-merle is recessive, and denoted by m.
Merle is pretty unique because all normal merles are heterozygous (Mm or mM). A homozygous merle is actually a double merle, similar to the lethal white gene found in horses and other mammals (although not itself lethal). Click here to go to the page on double merle.
The merle gene dilutes random sections of the coat to a lighter colour (usually grey in a black-pigmented dog), leaving patches of the original colour remaining. The patches can be any size and can be located anywhere, unlike the patches on a piebald dog (which are generally confined to the body and head). The edges of the patches may appear jagged and torn.
Merle affects only eumelanin. That means that any black, liver, blue or isabella in the coat, eyes or nose will be merled, whether it's the whole of the body, a mask on a sable, shading, brindle stripes, or even a saddle. Phaeomelanin (red) is not affected at all and will appear as normal.
Health Problems Linked to Colour
There are a few colour genes which can occasionally cause health problems in dogs, most notably merle.
Double Merles and High Whites
A homozygous (or "double") merle is one with two copies of the merle gene, and this severely impairs its ability to make pigment, leaving large areas of the dog pigmentless (white). Pigment is actually necessary for certain parts of the body to function correctly, so lack of pigment can cause health problems.
Dogs with large amounts of white caused by the homozygous piebald allele (sp), such as Bull Terriers, Boxers and Dalmatians, can also have some of the same health problems as double merles, particularly deafness (which is a huge problem in Dalmatians).
Lack of pigment in particular parts of the inner ear can cause deafness, which can be unilateral (just one ear) or bilateral (both ears). It is commonly claimed that dogs with white ears are always deaf, but in fact it's been shown that whether or not pigment is visible on the outer ear does not affect whether or not the dog can hear. In other words, a dog may have coloured ears but still be deaf, and a dog with white ears will not necessarily have any problems.
The double merle gene can also cause eye deformities. This is because the location of the eye cells in an embryo happens to be the same place that pigment starts to appear. If there is a problem with the pigment, this can therefore affect the development of the eyes. Problems include irregularly-shaped pupils, subluxated pupils (not positioned in the right place), microphthalmia (abnormally small eyes, usually with impaired vision), and other, less visible abnormalities causing blindness and bad vision.
Lack of pigment anywhere on the dog can make the skin much more sensitive to the sun. This is a particular problem on the nose, as it is so exposed, but any area of pink skin is susceptible to sunburn and skin cancer. The same problem occurs with any animal which has little or no pigment. White cats are probably the most well-known example. Skin cancer rates in white cats are extremely high and a surprising proportion of cats with white ears end up having their eartips amputated to stop the spread of cancer. The main way to prevent sunburn in animals is the same as with humans - apply suncream!
Dilutes
There is a common misconception that dilutes are in some way naturally sickly - this is not in fact the case. The dilution gene does impair the ability of the cells to make pigment, but only in that it causes the pigment that is made to be less intense. As with most recessives, the dilute allele is in some way "faulty", but it is only faulty in its ability to produce full-strength eumelanin.
The ability or inability of the cells to produce full-strength eumelanin does not affect the health of the dog, simply its colour.
That said, the idea of dilutes as unhealthy most likely has its foundations in Colour Dilution Alopecia. This is an apparently genetic disease causing hair loss and skin problems. A dog with this disorder will typically appear "mangy" and have partial hair loss. It is usually reported from blue dogs, particularly Dobermanns, but presumably it affects isabella dogs too (diluted livers). Any colour can carry CDA or be homozygous for it, but only blues and isabellas will have symptoms.
CDA does not occur on all dilutes and its frequency varies between breeds. It is particularly common in Dobermanns, occuring in up to 80% of dilute dogs. Dilutes in other species such as mice are caused by the same gene, and yet CDA is not known in these, implying it is not an unavoidable consequence of dilution. It is thought that CDA may be caused by a specific dilution gene - labelled dl. Just as there are various different b alleles that all cause the liver colour (phenotypically the same, so only distinguishable through genetic testing), it is probable that there are a number of different d alleles as well, and only one of these causes CDA.
What this means is that CDA is most likely caused by a recessive allele but could theoretically be bred out of most lines by careful breeding and genetic testing.
This blue German Pinscher appears to have mild alopecia. Its coat is dull rather than having a healthy shine, and it seems thin and patchy.
The same problem can also occur (albeit rarely) on black or liver dogs, and is known as Black Hair Follicular Dysplasia. It affects black/liver hairs only, leaving all other hairs as normal. Because this condition is so rare, it often goes undiagnosed. I used to know a Jack Russell Terrier mix who was white except for a black patch on his back, which was hairless. His condition puzzled a whole string of vets and skin specialists, who suggested various types of mange and allergies, and he was never properly diagnosed as having Black Hair Follicular Dysplasia. Unfortunately for dogs with genetic hair loss conditions, there is no cure, although these conditions do not generally cause the dog to be itchy or uncomfortable and so are mostly harmless.