Dog Genetics – Basic Terms

All the different aspects of a dog’s coat like texture, length, color or pattern are controlled by a handful of genes, their variations and their mode of inheritance.

If you want to learn more about dog coat color genetics now is the time to freshen up some of the basics before we dive deeper into the inheritance of coat colors in dogs.

Genes & Alleles

Think of DNA as the instructional manual of your dog.

A single gene is a small unit of DNA that contains the information a cell needs to build a specific protein. The entirety of building plans in a dog’s DNA is called its genome.

All dogs have the same genes.

But quite obviously not all dogs are built the same. And that’s because different dogs can have slightly different versions of the same gene. These gene variants are called alleles.

Alleles are the result of mutations in a gene.

Not all mutations in a gene cause a visible change in phenotypic expression. However, if we look at coat color genes we are only interested in mutations that actually do change how our dogs look.

Example: Every dog has a Merle gene in its DNA. However, not every dog has one of the alleles (M*/m) that can actually cause a Merle phenotype. Most dogs only have the original non-merle alleles (m/m).

Genes come in pairs, one copy is inherited from the dog’s father and one from its mother.

So if we look at a specific gene we find a set of two alleles, one inherited from each parent.

Different genes and their alleles get assigned a unique letter to simplify things a little.

Every newly found sequence of DNA gets assigned its own “gene letter”. And different versions (alleles) of this gene get some superscripted descriptive letter or symbol.

Example: At the K locus we find alleles like K^Black, k^brindle and k^yellow that we simply call KB, kbr, and ky.

The list of all the possible alleles on a specific locus is called its allelic series.

The original gene variant (as it most typically occurs in nature) is dubbed the wild type. The alleles of a gene are derived from the wild type through genetic mutation.

Here you find a summary of all the color genes in dogs.

Locus

The DNA in every cell is split into different DNA molecules called chromosomes. Humans have 46 chromosomes, dogs have 78 chromosomes. You can read more about canine genetics here.

A gene locus technically means the actual physical location of a specific site within the DNA sequence (locus is the Latin word for “place”). The word “locus” can refer to the gene (e.g. K locus, A locus, E locus). Or it can refer to the site of a mutation within the gene (e.g. bd locus, bc locus, bs locus within the B gene).

Zygosity

If a dog has two exact same copies of a gene (e.g. B/B or d/d), it is homozygous at this locus. If it has two different alleles (e.g. M/m or E/e), it is heterozygous at this locus.

Example: The K locus has 3 possible alleles (KB, kbr and ky). A dog with KB/KB, kbr/kbr or ky/ky is homozygous at the K locus. A dog with KB/kbr, KB/ky or kbr/ky is heterozygous.

Different dog breeds act as separate sub-populations within all dogs. Purebred dogs are often limited to only some of all possible coat colors and display a relatively low grade of heterozygosity.

Example: Many breeds only come in black & tan (Rottweiler, Manchester Terrier, Beauceron, etc.). They are homozygous for black eumelanin (B/B) and the tan point pattern (E/E ky/ky at/at).

Genotype + Phenotype

The genotype means the genetic code, the list of genes and alleles.

Now, gene expression turns this into the visible phenotype.

If we talk about the phenotype, we discuss what we can see. For example, the breed standards of different dogs list the desired phenotype.

The phenotype is often described with very descriptive terms. Additionally, different breeds often use their own breed terms to describe nuances in coat color.

Be aware of the information gap when discussing a dog’s genotype vs. its phenotype.

Example: If someone tells you their dog is “brown“, it could mean solid liver or maybe a red or orange coat with a sable or maybe recessive red pattern. Or something else entirely.

Modes Of Inheritance

If a dog is heterozygous at a certain gene locus, it has two different alleles.

The different modes of inheritance affect how these alleles interaact.

And this controls what gets expressed in the phenotype.

Dominant And Recessive Genes

Many of the color alleles we discuss on this site follow a path of complete dominance: If one dominant allele is present it will completely mask the presence of the recessive allele.

Example: On the B locus, there are only two possible alleles (B and b). B represents the wild type allele and holds the information a dog needs to build black pigment. The mutated version b lost this information and causes brown pigment whenever there is no B present.

Since one intact B allele is enough for a dog to build black pigment B acts dominant over b. A dog with B/B or B/b will have black pigment. Only a dog with two recessive alleles b/b will have brown pigment.

We know all dogs with black pigment have at least one B. We can not know if they are homozygous or heterozygous, so we simply write B/-. The “-” is a placeholder for the unknown second allele.

Since the presence of the recessive allele is hidden in the phenotype we call heterozygous dogs a carrier of the recessive trait. A B/b dog expresses black, but it carries brown.

Incomplete Dominance

In some cases, a heterozygous genotype can lead to a blend of both traits or an intermediate phenotype. We call this incomplete dominance or partial dominance.

Here, an allele can fully mask the effect of the other allele.

Instead, the interaction between two different alleles leads to a dosage-dependent effect and causes visibly less expression of a trait in dogs that have just one copy.

Example:

There are two possible alleles (S and sP) at the S locus. A dog can be either S/S, S/sP, or sP/sP.

S represents the wild type allele for a solid coat.

The mutated version sP causes white markings.

Homozygous dogs (sP/sP) have, on average, more white than heterozygous dogs (S/sP).

Another example of a color gene that acts as an incomplete dominant trait is Merle.

The impact of Merle is dose-dependent with two copies having a stronger effect than one copy (and longer alleles in these combinations causing more effect than shorter alleles).

Co-Dominance

If both alleles of a heterozygous genotype are equally visible and don’t mix to create an intermediate phenotype we speak of true co-dominance. Both alleles get expressed independently in the phenotype.

One example of this is the AB blood type where both A and B proteins are expressed at the same time.

Epistasis

We only call it dominance when we discuss alleles of the same gene, e.g. “B is dominant to b”.

We call it epistasis when we discuss the interaction of different genes.

Some genes can mask or interfere with the expression of other genes.

Example:

Every dog has some pattern at his A locus.

However, this pattern will not be expressed if he also has a KB allele at the K locus. The K locus is epistatic to the A locus. And the effect of both the A and K genes can be restricted by the E locus.

Modifier Genes

If a gene affects the phenotypic expression of another gene we call it a modifier gene. This is a form of epistatic interaction (one gene affecting the expression of another gene).

Modifier genes require another gene whose effect they can modify.

Example: Harlequin in Great Danes acts as a merle modifier.

Many modifiers are not testable. Modifiers are also responsible for the amount of dark overlay if a dog is sable. Or the size of a mask if the dog has one. Or the amount of white if a dog is piebald.

Further Reading

If you want to learn more, there are plenty of great resources available for free!

• Genetic Science Learning Center: Basic Genetics.
• The Useful Genetics Project: Course Material.
• Khan Academy: Mendelian genetics.

Learn More