Page 96 - JRT Magazine - 2023 Issue 2

Education

We have three possible genetic formulas for the can all occur, and all must be acceptable in Russell
K locus in JRTs: Terriers.

• KbKb – homozygous for dominant black gene. In most cases, locus K is fully dominant over
This dog will be black & white itself and will locus A, which means that the expression of
always produce solid black progeny regardless red/tan/yellow phaeomelanin as coded by locus
of what allele the other parent contributes. A would only be visible on dogs that are kyky -

• Kbky – heterozygous for dominant black gene. homozygous for the absence of dominant black.
This dog carries one allele of dominant black But some exceptions have been found. The
and the other one for its absence. A JRT with genetic reason is unknown currently but some
this formula appears solid black itself, and dogs that test heterozygous in the K locus (Kbky)
may produce, depending on the partner, solid and homozygous in the A locus (AyAy) appear to
black or tan progeny. exhibit the effect of co-dominance between these
two loci. This allows for the phaeomelanin (red/
• kyky – homozygous for absence of dominant tan) color hidden under the dominant black to be
black. This JRT is not bicolor black itself and it partially expressed by ‘leaking’ through it. As a
can only produce solid black puppies if bred to result, the colored parts of the JRT appear to be
a bicolor black & white JRT. a ‘dirty brown’ color, referred to as seal coloring.
The dog does not appear fully dark black, even
if genetically, the dog is dominant black and can
produce solid black puppies.

A bicolor black/white JRT is either KbKb or Kbky. Recessive
black does not exist in this breed.

We now see that if we want to have black &
white bicolored Russell Terrier puppies, at least
one of the parents must be black & white itself.
In our breed, it is impossible that solid black
(bicolor black & white) color will ‘pop out’ through
generations as is seen for tricolor dogs. Some
luck also is needed if just one of the partners is
dominant black and is heterozygous, because the
number of black puppies in this case can vary
from zero to a whole litter, depending on which
alleles are transmitted to the progeny.
Seal coloration may occur in a dominant black genotype. It
Another consideration when discussing the appears to occur primarily in dogs heterozygous at the K locus
while homozygous at the A locus (Kbky, AyAy) with some co-
dominant black Kb color is that some modifications dominance effect allowing the tan to seep through the dominant
are possible. Seal coloring, ghost tan, and bronzing black.

96 | A Worldwide Magazine for the Jack Russell Terrier / Russell Terrier • Issue 2/2023