MERLE COLORING.... Demystified.

What is Merle?

  Merle is a dilution gene, that lightens whatever coat color would otherwise have been.  The lightening is not spread evenly over the coat, but leaves patches of undiluted color scattered over the dog's body. 

  The gene responsible for the merle actually encodes for a protein that aids in the passing of pigment up the hair shaft.  With the presence of the Merle gene, some of the passage of the protein is disrupted, causing the variance in coloring of the hair shaft.

  There are always two copies of any gene.  If we call the Merle gene M and the non-Merle gene m, then any given dog can be mm, Mm, or MM.  the mm dog does not carry the Merle gene in them so will not display any Merle coloring, or genetic ability to pass it on.  The Mm carries the Merle coloring gene as half of its genetic makeup, so will show Merle coloring to varying degrees and carries the genetic ability to pass it on.  The MM is considered a double Merle or homozygous Merle.  This is where the concern can lie.  If a double Merle displays mostly white characteristics, especially in the eye and ear area, they are usually born either blind or deaf or both.  The reason for this is that in fetal development, the precursor cells for nerve and for pigment cells are one in the same.  As the fetus develops, the precursor cells migrate from the spinal column of the embryo in the direction of the head, so a dog with fewer pigment cells on it's head (excessive white) will have a greater likelyhood of having fewer nerve cells associated with the sensory organs of the head (eyes & ears).   The homozygous Merle does not always have health problems associated with the MM, but can be identified by excessive white present.  There are perfectly healthy MM dogs, but it is risky to breed for homozygous Merles and should be left to professional breeders.

Merle color varieties:

  The presence of the Merle gene on a black dog produces a Blue Merle, a more or less bluish gray coat dappled with black spots.  Tan points - the tan spots over the eyes, on the sides of muzzle and on the legs - a tri colored dog, will still be there.  The Merle gene does not act on the tan points.

   Merle on a Chocolate coat coloring, will react similarly with the dappling and spots.  There are many different shades of Chocolate, so make an interesting coat pattern.

   The Sable coat coloring will produce a Sable Merle.  This cross is less predictable than the Blue or Chocolate Merles and may range from an apparent solid Sable, often with a pinkish or orange cast to the coat, to something that looks like a rusty Blue Merle.

   Merle can also acts on the black pigment of the iris of the eye just as it does on the coat.  So often, merle dogs can have a part or all of the eye blue.  This does not affect their vision.

Tweed:

 There is a gene that modifies the Merle pattern, the "Tweed".  Tweed is a dominant gene that only expresses itself in the presence of a Merle gene.  The Tweed Merle has extremely varied Merle pigmentation.  For example, a Blue Tweed will have black spots, charcoal spots, slate blue spots and light blue spots.... even dark brown spots.  Red Tweed also exist, but are rare.  The Tweed patterns are often remarkably regular and the result can be stunning.