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A
certain cell type is responsible for the colour of the skin and coat:
the melanocyte. Inside these melanocytes the formation of pigment
granules takes place. These granules consist of the actual pigments:
eumelanin and phaeomelanin. Eumelanin is responsible for the darker
colours such as black, blue, chocolate brown and phaeomelanin causes
the lighter yellow/red colours such as tan, red, yellow.
The melanocytes originate in the embryo from the so called neural
crest. At a certain point in the development of the embryo these cells
migrate to various parts of the body where they will cause the colouring
to show. Also various sensory cells (e.g. from the eye and ear) originate
from this neural crest. A number of genetic defects of the sensory
organs is connected to certain colours and patterns, because of problems
with this migration.
Inside the melanocyte a number of mechanisms control the formation
of pigment. A couple of enzymes catalyse the chemical transformation
from tyrosine to the resulting pigment. The rate of activity of the
enzymes is controled through receptors in the cell membrane.
The genes that determine the coat colour can influence all of the
processes mentioned above (plus a few other processes). The method
of interaction of the individual genes (loci) will be explained below
if possible.
ely white Borders usually are swsw (except for double merles).
It is very likely that a number of modifier genes causes the variation
in markings. Since the variation caused by these modifiers is rather
extensive the various markings blend into another marking.
A (Agouti)
This locus controls the distribution of pigment in an individual hair.
This gene acts in the hair follicles where the melanocyte can switch
between eumelanin and phaeomelanin (dark pigment and tan pigment).
The actual switching mechanism is unknown, but is situated in the
dermis and for some alleles in the epidermis. The agouti colour does
not occur in the Border Collie as far as I know.
The sable colour is rather rare and does often not show the classical
effect of a dark dorsal area with a lighter abdomen. The hairs have
dark ends. Some authors suggest that Ayat dogs have a darker colour
than AyAy dogs. In mice however AyAy is supposed to be lethal (prenatal).
The at allele together with white markings produces the tricolour
pattern. The transition of tan coloured hairs to dark hairs is very
sudden. Tan markings occur in certain areas: above the eyes, on the
cheeks, at the shoulder joint, the front and innerside of the elbow
and the lower arm, the front and innerside of the knee and lower leg
and below the tail. The dog must be homozygote atat) to have tanpoint
markings.
Many authors also mention the saddle pattern. The dark colouring is
concentrated in a saddle-like patch on the back. On the head one can
also find black markings. The pattern is not as specific as the tricolour
markings.
Alleles
A fully coloured
Ay sable
asa saddle
ag Agouti
at tricolour or tanpoint
B (Black/brown)
This locus encodes the Tyronsinase related protein 1 (Tyrp1), which
catalyzes a quite distal step in the synthesis of melanin. The eumelanin
granules in bb animals are brown instead of black and shperoid instead
of ovoid.
Alleles
B black
b brown
D (Diluted)
The D-gene determines the shape of the melanocytes. In dd animals
the pigment cells are misshapen, the pigment granules are fewer and
larger, and the cells have fewer and thinner dendritic processes.
The result is that the colour looks paler, although melanin production
is not affected, nor is the microstructure of the pigment granules.
Black becomes blue, chocolate brown becomes lilac.
Alleles
D not diluted
d diluted
E (Extension)
The activity of tyrosinase is controlled by a few receptors in the
cell membrane of the melanocytes (so called melanocortin peptide receptors);
one of which is MC1R. These receptors bind melanocyte-stimulating
hormone ?MSH and other melanocortin peptides. The extensions gene
interacts with the agouti gene through these receptors: Ay dogs have
a defect in the synthesis of ?MSH, resulting in phaeomelanin becoming
visible.
Dominant alleles cause the extension of eumelanin and recessive alleles
the extension of phaeomelanin
Alleles
E eumelanin present
e eumelanin surpressed
M (Merle)
This
gene has acquired quite a bad name. If this dominant gene is present
only once, the base colour changes into an irregular pattern of various
grades of dilution. If the dog has two copies of the allele (homozygous),
the dilution is much stronger. The often (nearly) white animals can
suffer from blindness, deafness and/or be sterile.
This type of dominance is called 'incomplete dominant'. Although a
single Merle gene is visible the effect is fully visible in a homozygous
animal.
To the best of my knowledge there are no adverse effects in heterozygous
animals which makes the colour safe to use (if no Merle to Merle matings
are used). In general there should be no fear of 'carriers' for this
gene since the effect is easily visible. In very rare cases a dog
can look like a non-Merle, while he is genetically a merle ("Phantom
merle"). When such a dog is mated to a merle part (25%) of the
pups will be homozygous for merle. In Sable Merles the merle pattern
is actually only visible in pups. Later on the effect is nearly invisible.
To be on the safe side one should not mate sables to merles.
Alleles
M Merle
m not merle
S (Spotted)
Controls the distribution of colour on the body. Various authors are
rather unclear when it comes to describing the different patterns.
S is hardly present in the Border Collie and sw is rare. A few authors
mention that sp causes a rather irregular pattern. Probably si is
the most common in the Border Collie.
Extremely white Borders usually are swsw (except for double merles).
It is very likely that a number of modifier genes causes the variation
in markings. Since the variation caused by these modifiers is rather
extensive the various markings blend into another marking.
Alleles
S totally pigmented
si irish spotted/collie marked
sp piebald spotted
sw extremely white — only ears and base of tail is coloured
T (Ticking)
The white parts of the dog contain freggles. This causes the mottled
effect in the Border.
Accordign to some authors this gene show intermediary inheritance,
so TT animals are heavier ticked than Tt animals.
Alleles
T Ticked
t no ticking
Genetic overview of coat colours
This table tries to match the various colours (phenotypes) with the
present genes (genotyp). Is in the presence of a dominant allele the
other allele is unknown or without influence this is denoted by a
minus-sign.
Locus |
A |
B |
D |
E |
M |
S |
Black/white |
A- |
B- |
D- |
E- |
mm |
sisi |
Blue/white |
A- |
B- |
dd |
E- |
mm |
sisi |
Chocolate
brown/white |
A- |
bb |
D- |
E- |
mm |
sisi |
Lilac/white |
A- |
bb |
dd |
E- |
mm |
sisi |
Yellow(red)/white |
A- |
B- |
D- |
ee |
mm |
sisi |
Sable/white |
Ay- |
B- |
D- |
E- |
mm |
sisi |
Saddle |
asaasa |
B- |
D- |
E- |
mm |
sisi |
Tricolour |
atat |
B- |
D- |
E- |
mm |
sisi |
Chocolate
brown/tri |
atat |
bb |
D- |
E- |
mm |
sisi |
Blue-tricolour |
atat |
B- |
dd |
E- |
mm |
sisi |
Blue
merle |
A- |
B- |
D- |
E- |
Mm |
sisi |
Red
merle |
A- |
bb |
D- |
E- |
Mm |
sisi |
Black/white
(Moritz) |
Blue/white
(Escorial Blue of Forestry Farm) |
Tricolour
(Big Boss of FF) |
Chocolate
brown
(Deep Love at Wendigo's Wind) |
Blue/tricolour
(FF Honey Babey) |
Lilac
(blue/brown)
(Bob from Shadow of the M.) |
Yellow/white
(Rhonabwy Awsome Lady) |
Yellow/white
and blue
(Rozi) |
Blue
merle (Dunnorthac Woosh Its Blue at Rannaleroch) |
Blue
merle
(Japanese Girl at R. P.) |
Blue
merle-tricolour
(Calm Before The Storm at W's W) |
Slate
merle
(Athos Druna Blue)
|
Short
hair
(V'Nevian
MacLeod Silvertip) |
Red
merle
(Elite of Maranns Home) |
|
|
Photos:
the owners and Gábor Szalánczi