There are better was to explaining this but here's a little information.
There is no such thing as a sable merle gene or a red or blue merle gene.
There is only a merle gene. Merle is an INCOMPLETE DOMINATE gene, which
means
this gene will show up in it's offspring. Merle is also a dilution gene,
that is, it lightens whatever the 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. Also, the lightening seems to
work primarily on the black pigment in the coat, so any tan on the face
stays even.
Note that "black" as used here includes the brown color.
One dose of the merle gene on an otherwise black dog produces a
blue merle - a more or less bluish gray dog dappled with black spots.
Tan points - the tan spots over the eyes, on the sides of the muzzle, on the
legs
and under the tail of a tricolor dog - will still be there in the merled
tricolor.
If the tan spots would not be present in a black dog, tan will not be
present
in the merled black either, and the dog will blue merle and white without
tan.
One dose of the merle gene on an otherwise sable dog produces a sable merle.
Sable merles are less predictable in color than blue merles, and may range
anywhere
from an apparent sable, often with a pinkish or orange cast to its coat, to
something
that looks like a very rusty blue merle. White markings remain on the merled
dog,
and may even be slightly more prominant.
Merle acts on the dark pigment in the iris of the eye just as it does on the
coat, so merle dogs often have part or all of the eye blue. (This does not
affect their
vision, though since it happens to some extent in the retina as well it may
make it harder to diagnose certain eye problems.)
Notice what was said was a single dose of the merle gene. There are always
two copies of a gene, alike or different, in any dog. If we call the merle
gene M and the non-merle gene m, any given dog can be mm, Mm or MM.
The mm dog is the normal, full-colored tri, bi-color, or sable, or liver
(red) in
Australian Shepherds. The Mm Cocker is a blue merle, or brown merle,
etc,depending on
what color it would have been without the merling gene. An MM dog,
often called a double merle or a homozygous merle, will be mostly
white and sometimes deaf or blind and often with other physical problems.
Some MM puppies are born completely without eyes.
On average over a large number of litters, breeding merle to merle will
produce
one fourth full colored dogs, one half merles and one fourth defective
whites.
Breeding merle to full color will produce one half full color and one half
merles,
but no defective whites. The merle to full color breeding, then, produces
just
as many merles as does the merle to merle breeding, and without the danger
of
defective puppies. The safe breeding for a merle, then, is to a non-merle
mate.
This breeding should produce all healthy puppies, and about half will be
merles.
To breed in this way, it is important to know which dogs are merles.
This is one of the reasons experienced breeders rarely
breed blue merles to sables,
or roans as this mating may produce sable merles or merles to the
inexperienced breeder to appear incorrectly as roans..
Sable merles are no more likely to have health problems than any other
color.
Sometimes you cannot tell a sable is a merle unless you see it on the tips
of it's
ears or the eye or eyes are blue or blue flecked.,. The real arguement
against sable merles is that they may be mistaken for normal sables. If two
such sable
merles were mated together, the resulting litter could contain defective
whites.
The sable gene can also mask the merle, and as the dog ages it may be
next to impossible to tell by looking that this IS a merle dog.
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