Canine Coat Colour Genetics & the Miniature Schnauzer Presented by Catherine McMillan
Understanding Pigment In the bottom layer of the skin's epidermis are cells called melanocytes. Melanocytes are derived during fetal development from the neural crest. Neural crest cells also form nervous system, adrenal gland, skeletal/connective tissues of the head. Genes that affect colour may also have effects on vision, hearing and other processes.
Melanocytes produce melanin, the pigment found in the hair, eyes, and skin. Melanin is also a "photoprotectant" absorbing harmful UV radiation. Variations in pigmentation relate to the number, size and distribution of melanosomes, specialized structures within the melanocytes that inject tiny pigment granules into the hair shaft as it is formed in the follicle. Melanin occurs in two forms: Eumelanin - produces hair colours varying between reddish brown - black. Phaeomelanin - produces hair colours varying from yellow to red.
For the purpose of simplicity, I'll be refering to these two pigments as black and tan. Eumelanin black Phaeomelanin tan The rainbow of colours that occur in purebred dogs are the result of differing genetic combinations whose chemical interactions alter the intensity, patterning, and distribution of these two pigments. In this way, black pigment is altered to brown, tan pigment can range from red to nearly white, and so forth.
Review The Genetic Basics Genes are arranged into chromosomes. Chromosomes function in pairs (homologues), one inherited from each parent. Chromosome pairs from a human cell.
There are 39 pairs of chromosomes in the canine genome, (for a total of 78). Each gene occupies a specific locus on its chromosome and codes to produce a protein or function, known as a trait. There may be many variations (alleles) of a gene, resulting in varying expressions of that trait. The genetic makeup of an individual is called the genotype. The observable traits that are expressed in that individual is the phenotype.
There may be many alleles available in a breed's gene pool, but no individual carries more than two. Alleles that code for identical expression of a trait are homozygous. Alleles that code differently are called heterozygous. heterozygous homozygous
When only one allele in a heterozygous pair is expressed in the phenotype, we say it is dominant over its recessive partner. genotype phenotype Co-dominance, incomplete dominance, & intermediate expression result in a shared or delayed expression of traits. genotype phenotype
Genes can express differently from dog to dog, even when arranged in identical combinations. Mm Mm Two examples of heterozygous merle shelties.
In addition to genetics, distribution of pigment involves randomness and environmental influence. Even these cloned cattle have different white markings.
In rare cases, mismarks and spots can be caused by somatic mutations. During this yellow lab's embryonic development a skin cell mutated from ee (yellow) to Ee (black). All the descendants of that mutated cell produce black pigmentation. Somatic mutations of this type are not heritable.
Many colours and patterns humans select for are the result of chemical processes that have profound effects on physiology. Extreme white spotting is the result of genes that reduce the number of neural crest cells a process that can also lead to deafness. Dominant yellow agouti genes in mice have been associated with obesity, diabetes and tumour susceptability.
Learn Your Colour Alphabet The colours we see in canines, wild and domestic, are the result of interactions of many gene pairs located on different chromosomes. Some breeds have many colour alleles available in their gene pool, others have very few. All dogs, of all breeds, have two alleles at each colour locus, regardless of the color that is expressed in the phenotype. I will indicate which of these genes have been identified, while the rest of the information presented here is based upon published theory and personal observation. In addition,there are almost certainly other loci that affect colour intensity, hue, and patterning in more subtle ways that have not yet been identified.
A is for Agouti The Agouti series has been mapped to chromosome 24. These are switching/patterning genes that control when and where eumelanin (black) and pheomelanin (tan) pigments are deposited in the hair. The Agouti series has several alleles. Mouse studies indicate dominant mutations produce more tan (yellow) in the coat, while recessive mutations produce more black. The primary alleles on this series, in order of dominance: Ay (yellow) > aw (wild) > at (tanpoint) > a (solid black) The genes on this series have now been mapped.
Ay - sable aw - wild Agouti patterns. In general more tan is dominant to more black. At - tanpoint Aa recessive black
The pattern we call salt & pepper has been tested at the agouti locus, and proven to be "wild colour". Research published in 2011 revealed that all Mini Schnauzers tested black, s/p and b/s were homozygous aw wild at the agouti locus. If this research is valid, it means that the black & silver is not a true tanpoint, but a pattern governed by genes at yet another, unidentified locus.
For the purposes of this discussion, I've designated this as yet unidentified locus as V for black and silver. awaw at the agouti locus (proven) plus vv at the black/silver locus. (theory)
The factor that creates black & silver also appears to express partially in the heterozygous state, in at least some dogs. So, while we think of black and silver color as a recessive, this may not be entirely true. The darker cast of some heterozygous dogs may be due to partial expression of v, causing a delay in the agouti switching process from black tip to tan band as the hair shaft grows. awaw VV awaw Vv awaw vv
B is for Brown Brown dilution has been mapped to chromosome 11. It alters the structure of black pigment, converting it to brown. All black pigment is affected, including nose leather and pads. The dominant B of this series is best thought of as "normal for black", while recessive b is "brown". B (black) > b (brown) Responsible for chocolate Labs, red Dobermans, etc. Not part of the legitimate schnauzer gene pool, though liver schnauzers can be found in the AKC registry. Commercial tests are available.
C is for Chinchilla Chincilla series - thought to dilute pigment of all types. Produces albinism. Known in mice, cattle, and humans but recent studies have found no genes in this series that affect canine coat colour. It's included here only to update those who have seen it mentioned in earlier writings.
D is for Dilution Causes dilution of both black and tan pigment, but changes to black are more noticable. Present at birth, affects nose, eye and skin colour. The dominant D of this series is best thought of as "normal", while recessive d is "dilute D (normal) > d (dilute) Mapped to chromosome 25, though mutations vary from breed to breed. (Commercial tests are breed specific.) Not known to occur in schnauzers, but if in doubt, nose colour will clearly distinguish a blue from a faded black.
E is for Extension Mapped to chromosome 5, this gene affects black pigment in the hair. The three known alleles, in order of dominance: Em (mask) > E (normal) > e (non-black) Em - facial mask includes Leonberger, pug, mastiffs and others. The allele is unlikely in schnauzers. E - normal production of black pigment throughout the body. EmEm Leonberger e - recessive mutation that produces a non-functional receptor which results in the inability of the hair follicle to produce black pigment. ee dogs are unaffected by genes on the Agouti series.
Recessive ee dogs cannot create black pigment in the hair, but can range from rich red (Irish Setter) to golden (yellow lab) to pure white (white poodle). Research has proven that "e" is the gene responsible for the white miniature schnauzer. A commercial test is available to identify Mini Schnauzers that carry this recessive.
G is for progressive Greying Dominant over non-greying. Recently mapped in horses, research in dogs is underway. G (greying) > g (non-greying) Greying affects both pigment types. GG may produce more pronounced effects than Gg. Greying occurs as the follicle slows production of pigment, resulting in pigment free hairs.
G - greying is almost certainly present in the schnauzer gene pool the factor responsible for progressive greying of all colour varieties. Greying genes are unrelated to the genes that create the basic colour varieties known as black, salt/pepper, and black/silver. Greying of blacks and black/silvers is not caused by crossing the colour varieties.
I is for Intense A new locus proposed that dilutes only phaeomelanin (tan), and leaves black unaffected. I (dilute intensity) > i (normal intensity) Creates the difference in the red tones in the wild boar dachshund and silver of the schnauzer. Schnauzers are homozygous for this trait.
K is for black Dominant black is the most common form of black in dog breeds, and has been mapped to chromosome 16. There are three alleles on the series: K (solid black) > kbr (brindle) > k (allows tan ) K is the allele responsible for black schnauzers. Salt&pepper and black&silver are kk at this locus. A genetic test exists to distinguish Kk dogs from KK homozygous blacks. Brindle is not known to be present in the schnauzer gene pool.
M is for Merle M (merle) has been mapped to chromosome 10. M (merle ) > m (normal) A dominant (or co-dominant) gene that creates alternating patterns of dark and light pigment. Common in herding breeds, also called dapple in dachsunds. Merle is a semi-lethal gene, and MM homozygotes are mostly white in colour, usually deaf and sometimes blind. Not a part of the legitimate gene pool in schnauzers. Commercial tests for merle are available.
S is for Spotting Spotting patterns are the subject of ongoing research. Best thought of as a lack of pigment. Spotting in boxers and bull terriers has been mapped to chromosome 20, where spotting is co-dominant with solid, with heterozygotes exhibiting flashy white markings. Homozyotes are extreme white, with high risk of deafness. Extreme white of this type is not known to be present in the schnauzer gene pool.
Piebald is another type of spotting that is recessive to solid. Mapped to chromosome 20, there may be several alleles, with "more colour" generally dominant to "less colour". S (solid) > sp (piebald) This is the form of particolour found in miniature schnauzers registered with the AKC, but disqualified under the standard. Welsh terrier puppies, from champion parents, both recessive carriers for piebald type spotting. A commercial test exists for some breeds.
Irish spotting - May be part of the spotting series mentioned above. If so, this pattern is recessive to S - solid and dominant to sp - piebald. Not found in the legitimate schnauzer gene pool. White chest spots, toe tips Melanocytes migrate from the spinal column downwards during fetal development, and delay of the process may result in small white areas on the chest, toes or underbelly. Not considered to be an inherited spotting trait, though the rate of melanocyte migration may be. Common in most solid breeds, including schnauzers.
T is for Ticking Ticking is the presence of small pigmented spots on unpigmented white areas. Dominant over non-ticking. T (ticking) > t (non-ticking) Common in sporting and hound breeds, there may be multiple variations of this gene. Dalmatian spotting may be a variation of ticking, or spotting of a different type. Not likely to be in the schnauzer gene pool, though it may be present, as ticking is not visible in S solid dogs.
sable brown particolour A few members of the illegitimate AKC gene pool Found advertised as purebred on the internet. salt&pepper particolour
Coat Colour In The Miniature Schnauzer Knowing the loci and the alleles that control colour in dogs is only the first step. In addition to the relationships between gene pairs at each locus, one must also factor in the relationship of these pairs in relation to genes at other loci. With dominance, one allele of a gene masks the expression of another allele of the same gene. Epistasis is when genes at one locus mask the expression of genes at a second locus a genetic trump card. In some cases, epistasis occurs when one copy (eg. dominant black) of the masking gene is present, in others, two copies (eg. recessive white) are required.
All colour begins with base "black" and "tan" pigment. A - Agouti series determines where and when black and tan pigment is deposited. To date, the only alleles found in miniature schnauzers at this locus are aw (agouti wild) awaw
ii ii Genes at the Intense locus dilute tan pigment only. All schnauzers are homozygous for this allele (including blacks). This gene appears to act as a dominant, or partial dominant. II II
The K locus (dominant black) is epistatic to the Agouti locus. Dominant black turns off agouti pigment switching. KK awaw -- Kk awaw -- kk awaw -- KK awaw vv Kk awaw vv kk awaw vv
G - progressive Greying. Dogs with two copies of this dominant gene seem to express more strongly than heterozygous dogs. KK awaw v- gg kk awaw -- gg kk awaw vv gg KK awaw v- Gg kk awaw -- Gg kk awaw vv Gg KK awaw v- GG kk awaw -- GG kk awaw vv GG
E - Extension Schnauzers with two copies of recessive "e" are incapable of creating black pigment, and are white/cream. KK awaw -- EE KK awaw -- Ee KK awaw -- ee kk awaw -- EE kk awaw -- Ee kk awaw -- ee kk awaw vv EE kk awaw vv Ee kk awaw vv ee
Those Other Colours...? Liver or chocolate is due to the presence of recessive brown dilution, which converts all black pigment to brown when present in the homozygous state, including that of nose leather and pads. Dogs who create normal black are BB or Bb. Dogs who are brown or liver are bb.
In addition, the AKC registers particolour patterned miniature schnauzers. As this gene affects only pigment distribution, the coloured portions may be of any found in the gene pool (including ee white). KK awaw -- ss black parti kk awaw --- ss salt&pepper parti ------ vv ss liver/silver parti Small white splashes on the chest, white toes and chin hairs on dogs that are solid coloured are not due to spotting genes, and do not necessarily indicate the presence of a recessive particolour allele.
A Word On Colour Crossing Some believe that colour crossing" leads to greying of the black varieties and muddy salt & pepper banding. All black dogs (KK or Kk) also carry 2 alleles for salt & pepper agouti locus. It's not possible to "introduce" salt&pepper through colour crossing. The genes are already there.
When heterozygous Vv salt & pepper dogs are bred, their alleles segregate and are passed on to offspring as single copies, in their original form. The genes for salt&pepper are not altered by the presence of genes for black&silver their expression is altered.
Cross of SS solid schnauzer (awaw SS II) and spotted Jack Russell with red ears (ayay spsp ii). Ayaw at agouti, Ssp at spotting locus, Ii at intense (This dog faded to pale tan as an adult, evidence that Intense is dominant.)
In addition to the major gene series covered in this presentation, other genetic factors modify how they are expressed. Genes for metabolism, coat texture, density of pigment, as well as diet and environment also play a role in the final observed color of a dog. This is why we see such variety from family to family, even though the base colour genes are identical. Color genetics of the dog is under active research, and you can watch for new developments at the following website: http://homepage.usask.ca/~schmutz/ dogcolors.html