How do the A and K loci interact to show patterns?
The interaction between the A-locus (ASIP gene) and the K-locus (CBD103 gene or Beta-Defensin 103) is based on a strict genetic hierarchy. Specifically, the K-locus acts as a gatekeeper that determines whether coat patterns coded by the A-locus will actually be visible on the dog.
Here is exactly how these two genetic locations interact to show (or hide) coat patterns:
• The Dominant Black Override (K
B
): The most dominant allele on the K-locus is K
B
. If a dog inherits even a single copy of this gene, it will have a solid base color, such as black, blue, chocolate, or lilac. This dominant K
B
The allele completely restricts the A-locus, meaning the dog will not physically display any A-locus patterns regardless of what it carries.
• Expressing A-Locus Patterns (K
y
): In order for the patterns on the A-locus to be visually expressed, the dog must lack the dominant K
B
allele entirely. A dog with two copies of the most recessive K-locus allele, known as the K
y
K
y
genotype, will allow the A-locus traits to “show through”. When the K-locus allows it, the A-locus controls the switching between dark pigment (eumelanin) and red/yellow pigment (phaeomelanin) to display patterns like fawn/sable (a
y
), wild sable (a
w
), or black-and-tan points (a
t
).
• Creating the Brindle Pattern (K
Br
): The K-locus also contains a mutation responsible for the brindle pattern (K
Br
), which is dominant over the K
y
allele but recessive to the solid black K
B
allele. To visually express brindle striping, a dog must possess the K
Br
allele on the K-locus while simultaneously possessing the fawn/sable alleles (a
y
) on the A-locus. If the dog has the K
Br
allele but possesses pointed alleles (a
t
) on the A-locus, it will appear as a “brindle point”.
• Carrying Hidden Patterns: Because the K
B
An allele easily hides the A-locus; dogs with solid coats can secretly carry pattern genes and pass them to their offspring. For example, a solid black dog might have a K
B
K
y
genotype on the K-locus and an a
t
a
t
genotype on the A-locus. Breeders refer to this as a “genetic tan point”; the dog appears completely solid black but will consistently pass one copy of the pointed pattern to its puppies.