Page 90 - JRT Magazine - 2023 Issue 2

Education

Locus A or Agouti is named by a series

of ASIP alleles - genes that control the amount,
distribution, and switching off and on of the two
pigments in the hair follicle by blocking fully or
partially the production of black pigment. Locus
A is the most complicated locus. It controls a
large range of coat colors from clear light yellow/
red to solid black and everything in between.
Expression of locus A is possible only on dogs
Without the Sp allele, a Russell Terrier could look like the terrier that have kyky genotype in the locus K -
on the left. Add the white-spotting gene and we have a Jack
Russell Terrier, a tri-colored one in this case. It’s a black & tan homozygous for absence of dominant black. If
dog with white markings (dog on right). dominant K black is present, it will override the
A locus.
Another term for a white-spotted
dog is piebald. White in JRTs is not Until recently (January 31, 2022), only four
actually a color; rather it is the absence Agouti variants were known. These four variants
of pigment. So piebald technically of Agouti alleles were referred to as: Ay (Agouti
does not mean ‘white pigmentation’, it yellow), Aw (Agouti, wild type, or wolf color),
means a deletion of color—an absence at (Agouti with tan-pointed markings) and a
of pigment. This absence of pigment (recessive black). In JRTs, Ay was commonly
can have other effects. The absence of called sable or tan and the at as black & tan. The
pigment in a tissue layer above the iris Wolf color and recessive black do not exist in
of the eye will give a blue eye. Blue eyes JRTs.
have no health-related consequences.
The absence of pigment in the middle
ear, however, can cause deafness.

Pigment-related deafness is the most Agouti yellow, often referred
common cause of deafness in dogs. to as tan, red or sable.
For an animal to hear, sound vibrations
travel through the outer and middle
ear and are detected by the cochlea
and transduced by neural hair cells.
These transduced vibrations travel
via the eighth cranial nerve through
the head, eventually finding their
way to the thalamus and the primary
and secondary auditory areas of the
temporal lobe. The nerve cells of
the cochlea rely on high potassium
levels in the fluid surrounding them to
survive. One of the known functions
of melanocytes in blood vessels is to
maintain those potassium levels. When
those pigment-producing cells are
absent, such as in the case of piebald Agouti-shaded yellow has
animals, the auditory nerve cells die off, black tipped sable hairs.
resulting in deafness.

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