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Inheritance of White Markings
in Cardigan Welsh Corgis
Until very recently the commonly accepted model of inheritance
of white markings and spotting was that postulated by Little
(1957). After numerous observations and test matings, he
concluded that relevant genes were located in S locus,
consisting of four alleles.
S - solid colour. When the S allele is present in either
homozygous or heterozygous form, the dog is of solid colour.
White may be present on toes and possibly forms a small spot on
the chest. Solid colours are not common with show Cardigans,
though a dog with little or no white marking is perfectly
correct. It may be a bit more difficult for a judge to spot
such a dog, but when judging Cardigans one has to remember that
correct type, structure and movement must always be given
preference to colour.
si - "Irish spotting"; white is restricted to muzzle, head,
neck, chest, legs, belly and tail tip. Typically the dog has
white muzzle, white blaze on head, full or partly white collar,
white socks of different length, and white tail tip. The si
allele is recessive to S, but dominant to sp (below). This
pattern is very common with Cardigans.
sp - piebald spotting; the resulting pattern is white with
patches of colour. These patches vary greatly in size, shape,
number and location. Even though this pattern occurs within the
breed, it's not univocally accepted (see: standards).
sexp - this allele, recessive to all previously described, in
its homozygous form, results in white coloured dog. Small
patches of colour may be present around eyes and on ears, or
the dog may be completely white. In the latter case it may be
also unilaterally or bilaterally deaf.
In order to explain why white markings are so variable in size
and shape, Little suggested that plus and minus (+/-) modifiers
of quantitative mode of inheritance contribute to the dog's
actual markings. Breeding practice suggests that mating heavily
marked dogs together through several generations increases the
probability that excessive white appears among progeny.
Results of recent DNA studies, however, do not fit into
Little's model. The first gene, responsible for spotting
patterns, was identified in 2007. This is MITF (microphtalmia
associated transcription factor), which regulates
differentiation and development of melanocytes, and is also
responsible for transcription of different enzymes involved in
pigment production. Two different mutations in MITF gene were
detected in white boxers and bullterriers, and it is possible
that in heterozygous form they result in typical, flashy Irish
spotting. In other breeds, including border collies, same
markings are not inherited as a heterozygous pattern -
therefore are most probably caused by another gene. The exact
pattern and mode of inheritance of white spotting in Cardigan
Welsh Corgi (and many other breeds) are yet to be established.
The great variability of white spotting may be attributed to
length polymorphism in microsatelitar non-coding DNA sequences,
which were found to be longer in dogs with white markings.
Although non-coding themselves, these sequences may change the
expression of the major gene(s), responsible for the presence
of white.
Pigment development in the embryo
The roles of pigment cells reach far beyond pigmentation of the
skin and coat of the dog. Amongst others they also play a vital
role in developing sight and hearing, several brain- and
digestive functions. It is therefore very important to retain
basic pigmentation in all dog breeds, be it in the skin or both
skin and coat. But this being said there is a huge gap between
the health problems caused by complete lack of pigmentation,
and the decisions faced by breeders and judges concerning
Cardigans with large white markings. For our breed to start
experiencing problems related to poor pigmentation, one would
probably have to breed dogs with extensive white markings and
poor skin pigmentation for several generations.
This quote from the website www.ratbehavior.org covers the basics of
pigment cell formation and migration very well:
"In the embryo, a fold develops down the back called the
neural tube, which contains an active region called the neural
crest. This region supplies the pigment cells (melanocytes)
that migrate all over the body.
Specifically, the pigment cells migrate to pairs of specific
sites on either side of the body as well as the backline. There
are three such sites on the head (near the eye, near the ear,
and near the top of the head), and six sites along each side of
the body, and several along the tail. A few pigment cells
migrate to each of these sites, where they proliferate and
migrate outwards, joining up to form larger patches, spreading
down the legs and down the head until they meet up under the
chin, and down the body until they meet up on the belly
(Cattanach 1999).
Once the pigment cells have finished migrating they take up
positions at the base of hair follicles. There they synthesize
melanin pigment, and feed it into the growing hair. Normally,
all follicles have pigment cells associated with them and all
of the animal's fur is pigmented. But if no pigment cells are
associated with a follicle, there is no pigment in that hair.
Mutations that affect pigment cell distribution during the
development of the embryo determine which parts of the body
have pigment cells, and hence produce pigment, and which parts
have no pigment cells and produce de pigmented hairs.
Pigment cells also migrate to the iris and retina of the eye.
If the iris does not have pigment cells, it looks red (in rats
and mice) or blue (dogs and cats). Odd-eyed rats are caused by
the migration of pigment cells to one eye but not the other.
(Note: the white coat and red eyes of albinos are not caused by
a failure of pigment cell migration, but by the inability of
pigment cells to produce pigment).
Pigment cells migrate to the inner ear, too (cochlea and stria
vascularis), where they play an undefined but essential role in
maintaining hearing. If the inner ear does not have pigment
cells, the individual may be deaf.
Pigment cells also migrate to the brain, to areas such as the
substantia nigra (part of the midbrain that regulates mood,
produces dopamine, and controls voluntary movement), the locus
ceruleus (part of the brain that deals with the stress
response) as well as other areas such as the leptomeninges
(membranes surrounding the brain), the dorsal root ganglia, and
the cranial ganglia. The failure of pigment cells to reach
these areas can have a wide variety of effects, such as a
movement disorder (e.g. seizures), and diverse effects on
behaviour and the individual's response to stress.
Pigment cells are therefore implicated in areas of the brain
related to mood and the stress response. This connection
between depigmentation and behaviour probably played a role in
animal domestication. By selecting for tameness, breeders
selected for a different pigment cell migration in the
developing nervous system, leading to calmer animals. A side
effect of this selection for behaviour was the change in
pigment cell migration in the skin, leading to a piebald coat.
Piebaldness and associated docility are found in many different
domesticated species (horses, cows, dogs, cats, birds). In
fact, selection of wild animals for tame behaviour leads to de
pigmented areas on the fur, as shown in foxes (Belyaev 1979,
Trut 1999) and rats (Trut et al. 1997). Note, however, that if
depigmentation is extreme, the animal may have neurological
impairments (Grandin 1998)."
20.02.2008
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