Cardigan Colours

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 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)."


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