Page 89 - JRT Magazine - 2023 Issue 2

Currently, it is impossible to predict the exact
spreading of white spotting and so piebald dogs Alleles Sp and S are examples of
with very different proportions of white to colored co-dominant genes. In the case of a
patches will all have the same genetic formula. It heterozygous genotype at the S locus
is possible that the S allele exists in the JRT gene (i.e., SSp), both alleles will be expressed
pool, although S leads to minimal white markings but neither fully. Dogs with SSp will
or no white at all, with the result unacceptable for have more white than SS dogs that have
JRTs per the breed standard. minimal to no white, but they will have
less white than SpSp dogs.

We may assume the existence of other
alleles or modifiers at this locus that
could explain asymmetrical or split face
markings on the head color, extreme
white, or some specific patterns, but
nothing has been yet proven.

These two JRTs likely are not homozygous for Sp. It is a fault per
the JRT breed standard to be so highly pigmented. The amount of white can vary greatly no matter if the Sp locus is
homozygous or heterozygous for piebald spotting. The patterns
are unpredictable.

The color of a JRT is often written as
‘white with red markings’ or ‘white with
black and tan markings’ when in actuality,
the dog is genetically ‘red with white
spotting’ or ‘black and tan with white
spotting’. The base color is the pigment;
the white is the marking.
The Sp piebald spotting in JRTs usually fully covers the
underside and the limbs, but not always.

A Worldwide Magazine for the Jack Russell Terrier / Russell Terrier • Issue 2/2023 | 89