Page 88 - JRT Magazine - 2023 Issue 2

Education

Alleles hair. But with an absence of pigments, trapped air

Alleles are different versions of a gene, and molecules make the hair appear white, which is
the various versions exist in a hierarchy so that an effect similar to that for snow.
some alleles can be dominant, recessive, or co-
dominant in their interaction with their partner in Alleles located in the locus S are one of the
the gene pair. In a heterozygous locus, the trait regulators of pigment production and they give a
to be expressed will be the one coded by the ‘command’ for the hair follicles to produce or not
dominant allele. With co-dominant alleles, neither produce pigments in specific areas. All JRTs likely
allele has priority of expression and so both will are SpSp – homozygous for piebald spotting—to
be visible, sometimes partially. create big white areas as required by the breed
standard (i.e., predominant white) and often
means a minimum of 51% of white.

• SpSp – homozygous for piebald spotting. Dog
carries two copies of piebald-spotting gene.
The dog with this genotype at the S locus has
big white patches of various patterns.

GENOTYPE:
the genetic makeup or DNA
code of an individual.

PHENOTYPE:
the detectable expression
of the genotype plus
environmental factors.

In the purebred Jack Russell Terrier, the diversity
of colors possible in the phenotype exist by
expression of genes in eight loci: S, A, B, K, E,
D, T, I.

Locus S could be considered the most
important locus for the JRT since it is responsible
for the white. Without white, a terrier cannot be a
JRT/Russell Terrier. Two alleles exist at this locus:
S and Sp. The Sp stands for ‘white spotting’
or ‘piebald spotting’ since the base color of the
dog’s phenotype is covered by white in various
locations. Interestingly, white as a color, does not
actually exist. There is no white pigment. Rather, it
is the absence of pigment that makes a hair shaft
appear white. The cortex of a hair is transparent.
With a black dog, there is eumelanin pigment Piebald white spotting patterns vary, but the JRT should be at
inside the shaft, giving the appearance of a black least 51% white.

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