Homozygous "Lethal White" Merles
Homozygous Merle or Double Merle Australian Shepherds Suffer
Serious Health Problems
Why are Double Merles Called "Lethal
What Does "Merle" or "Double
What Health Problems Do Double Merles Have?
Are Pattern Whites the Same as Double Merles?
what's so bad about white Aussies? It actually sounds
quite nice, an Australian Shepherd with a snowy white coat.
The problems that arise have to do with the underlying genetics
that result in physical disorders rather than a concern about
appearance. It just so happens that the genes that are involved
in determining pigmentation or coloring also have a dramatic affect
on the development of hearing and sight.
Were it not for the health problems that result from this condition
white Aussies might have just been another coloration category.
However, due to the serious health repercussions too much white
is considered a defect on Australian Shepherds as far as conformity
to the breed standard is concerned.
This genetic defect occurs when two merle coloured dogs are bred.
It doesn't matter if they are blue merle or red merle the key
here is that the puppies inherit two dominant merle genes (MM).
This double whammy of the merle gene results in a homozygus merle
or double merle puppy often referred to as a lethal white.
Why are Homozygus Merles Called "Lethal
Allstar Paint Horse Ranch
Horse with Overo Coat Pattern
First I should mention that the phrase lethal white although it has become popular in
describing double merle Australian Shepherds has been misappropriated
from a condition that afflicts Paint Horses with overo coat patterns.
This condition is called Overo
Lethal White Syndrome > (OLWS).
However not all overo pattern Paint Horses are carriers of the
lethal white gene. Paint Horses with tobiano patterns, Quarter
Horses and other breeds can also be carriers.
In Paint Horses the foal is born entirely white or nearly all
white if it inherits the lethal white allele (DNA sequences that
code for a gene) from each of its parents. They are also born
with an abnormality affecting their colon that doesn't allow them
to pass feces. This condition is invariably fatal.
So, that's where the phrase lethal white came from. Okay, back
to Australian Shepherds.
Overo Lethal White Syndrome is not related to what happens to
homozygous or double merle Australian Shepherds, Mini Aussies
and many other breeds that carry the merle gene such as Border
Collies, Koolies, Great Danes and "dapple" Dachshunds.
Lethal white is a misnomer as it is not actually fatal in dogs,
unless you consider that puppies born double merle are often culled.
The only similarity is the white coloration of the coat, and with
Aussies this only rarely results in a completely white dog.
However the phrase lethal white has become popularized and as
such has become entrenched in Aussie vocabulary. Other phrases
often used instead of lethal white are excess whites, over merled,
merle merles, fatal whites, white merles and rare whites.
I prefer double merle rather than lethal white for a few reasons.
First of all lethal white is simply inaccurate. While on the other
hand double merle removes the implication that these dogs do not
live long which is a fallacy. Their lifespan is the same as any
Australian Shepherd. It also focuses attention on the "merle"
aspect which serves to educate as to the cause of this condition.
Believe it or not, but search engines may be responsible for
perpetuating the phrase lethal white. Since lethal white has become
a commonly used description of homozygous merles many websites
attempting to educate the public about lethal white merles are
forced to include the words lethal white on their web pages just
so those looking for information by typing in "lethal white"
into their favorite search engine can find them.
So, lethal white, lethal white, lethal white. There, that should
What Does "Merle" or "Double
Merle refers to the pattern in the coat and is not a color as such. The white and gray patterns
that appear on a black Aussies make them appear to have a blueish
cast. These are called blue merles.
Merling occurs in all the normal Aussie colours such as the red
merle. Again the merle is not the color of the dog it is the white
or lighter patterns between the darker areas.
In the case of a healthy merle the puppy has received the merle
allele from only one of its parents. For example breeding a black
bicolor and a merle. In the case of a double merle inheritance
the merle allele is passed from both parents and the puppy gets
a double dose.
Since the merle allele affects pigmentation a dog that would
have had patches of white or light areas with a single inheritance
becomes predominantly white with the double.
Inheritance of gene pairs can seem complicated to the non-breeder.
To simplify, each puppy recieves a gene from each of its parents.
Genes can be either dominant or recessive. Dominant genes mask
the affect of ressesive genes.
Say we have a merle Aussie who has a gene pair of Mm.
The capital 'M' represents the dominant merle gene while the lower
case 'm' represents the recessive solid gene. Because the merle
gene is dominant and masks the affect of the solid gene this dog
is a merle. This is called a "heterozygous merle.
Now suppose we bred this dog to another dog that was also an Mm. The puppies could get various combinations
of inheritance depending on which gene they received from each
Mm + Mm = mm or Mm or Mm or MM
mm - Solid
Mm - Heterozygous merle
Mm - Heterozygous
MM - Homozygous merle (Double merle - Lethal White)
So, in an Mm + Mm breeding the chance for a puppy inheriting
two dominant merle genes (M) is about 25 percent. These odds for
causing a puppy to be born permanently deaf and/or blind are too
This is why you should never breed two merles of any color. Breeding a blue merle with a red merle will make
no difference. When it comes to breeding merles with merles - JUST DON'T DO IT.
What Health Problems Do Homozygus Merles
The health consequences
vary from mild to severe affecting hearing and vision.
Some puppies are born deaf, others blind and in the worst cases
Deafness resulting from a merle to merle breeding usually develops shortly after birth. During this time the ear
canal is still closed.
The white coloration of double merles is the result of a lack
of pigment producting cells (melanocytes).
One of the functions of these pigment cells is to provide high
potassium levels in a fluid (endolymph) that surrounds the hair
cells of the cochlea. When there aren't enough of these cells
present a significant reduction to the blood supply to the cochlea
occurs. Specifically, the stria vascularis is affected. As a consequence
the nerve cells of the cochlea begin to die resulting in permanent
There is no treatment or surgery that can reverse this damage.
This is a lifelong condition that will have to be dealt with for
the 15 or more years of the the dog's life.
Another explanation that accounts for degrees of hearing loss
proposes that hair cells in the cochlea convert mechanical energy
to electrical nervous impulses that get passed on through the
cochlear nerve eventually making its way to the brain stem and
then the brain where it is interpreted as sound.
The breakdown in this chain of events occurs in the conversion
of the mechanical energy to electrical impulses. Pigmentation
cells are required for this process to occur.
The degree of conduction hearing loss is directly related to
the presence of pigmentation cells. If there are still some pigmentation
cells there will still be some hearing, if not the dog will be
deaf. This can happen in one or both ears.
Deafness in dogs poses a safety concern for the dog. For example
they may not be able to hear approaching vehicles, livestock or
other dangers until it is too late.
On the other hand a deaf dog also poses a saftey concern for
anyone who comes into contact with it. There is real potential
to be bitten if someone were to startle the dog. This would be
of particular concern with children who may not consider the dog's
deafness in how they approach. Children and deaf dogs don't really
This is not to say that these dogs are aggressive or have a bad
temperment. It's just that it is easier to catch them by suprise
and they may react by reflex as a result of being startled.
Homozygous merles can also have vision problems up to and including total blindness. Merle Ocular Dysgenesis refers
to a group of eye problems associated with merle to merle breeding.
This list includes iris
coloboma or other deformations of the iris, cataracts,
retinal detachment, persistent
pupillary membrane, lens luxation (displacement of lens),
equatorial staphyloma (protrusion occurring in the area of exit
of the vortex veins) and lack of a tapetum licidum (reflective
surface at the back of the eye) resulting in night blindness and
poor sight in low light conditions.
In addition to these problems there are the very serious and
generally more obvious conditions we discuss below.
The same lack of pigment cells that results in hearing loss affects
the development of the eyes. Eyes begin developing while the puppy
is still an embryo. Depending on when that development is arrested
the eye will suffer greater or lesser damage.
Sometimes the growth of the eye is impeded and one or both eyes
are smaller than normal (microopthalmia) or are missing altogether
The acuity of the vision in these smaller eyes varies with the
degree of deformity. In some cases the third eyelid permanently
covers the small (or missing) eye as well.
As if this wasn't bad enough there are other eye problems faced
by double merles.
Hypoplasia is a condition in which the iris
is not completely formed and pieces are missing. This can result
in squinting in bright light as the iris is not able to function
properly helping to reduce the amount of light let into the eye.
While this occurs in other dog's, double merles are particularly
suceptible to hypoplasia.
In some cases the pupil is not positioned properly in the center
of the iris. This is referred to as an eccentric pupil or dropped pupil. This can make it appear
that the dog is looking in a different direction than what they
A starburst pupil or sunburst pupil is where the pupil is not properly formed and looks as if it is
sending out rays into the surrounding iris. These dogs also have
a problem with bright light as the pupil is unable to contract
as it should to protect the eye.
Similar to the starburst pupil is the irregular pupil. Rather than being perfectly round the irregular pupil is rough
and jagged around the outside edge.
There is a myth that these eye problems only occur in dogs with
blue eyes. That is simply not true. It may be easier to spot in
blue eyed dogs due to the greater colour contrast but these problems
occur in dogs of all eye colors.
There is also a wide range of possible combinations. These conditions
can affect one or both eyes and more than one condition can be
present in the same eye. So you could have one eye almost of normal
size with an irregular pupil while the other eye is smaller with
a starburst and eccentric pupil.
I'll spare you the graphic and heartbreaking pictures here, but
if you would like to have a look you can find photos
of microopthalmia, anopthalmia, hypoplasia, starburst pupil, irregular
pupil and eccentric pupil here and here.
Impact on visual acuity is similarly varied ranging from mild
impairment to total blindness. As with dogs with hearing impairment/loss
always use common sense when approaching so as not to startle
them. You must excerise reasonable caution to aviod getting bitten.
Are Pattern Whites the Same as Homozygous
Although homozygous merles
and pattern whites can look similar, as they can both
be predomininatly white, the main difference is in what causes
the coat to be white.
With pattern whites or "Mismark Aussies" the underlying
cause is not the double inheritance of the merle gene (MM). It
is due to the influence of a white spotting gene that determines
white trim area, the S locus.
Alleles of the S locus such as the Irish Spotting, Piebald Spotting
and the Extreme Piebald Spotting are responsible for determining
which areas of the dog will be white.
S Self - completely solid color
St Irish spotting creates white trim on muzzle, blaze, collar,
belly, stockings and tip of tail
Sv Piebald spotting creates colored patches on white background
Sw Extreme piebald spotting creates very little color on white
Added into the mix are plus and minus modifiers that exaggerate
these effects and add or remove even more pigment than would normally
Degrees of white colour are determined by the permutations and
combinations of these genetic factors. From dogs with solid coloration,
to acceptable white trim, to excessive white (by conformation
standards), to only a few patches of color and finally to completely
Although the reason for the excessive amount of white is genetically
different than in the case of homozygous merles it can be difficult,
even for experts, to tell the difference just by looking at a
Does this mean that because of a lack of pigmentation that pattern
whites can suffer the same health issues as double merles? Yes,
if the lack of color occurs in the key areas around the ears and
eyes pattern whites are at risk of having hearing and vision problems
Pattern white puppies can be born of parents who are not merle.
They may be more likely to occur with parents who already have
excessive amounts of white in their coats but that is not always
the case. It is good to be aware of the health implications of
a lack of pigment particularly around the ears and eyes.
The genetics involved can be very complex with both double merles
and pattern whites. This is another reason why it is so important
to leave breeding to the experts. Reputable Australian Shepherd
breeders who care about the breed and their dogs take care to
avoid these pitfalls. Combined with veterinary care and assessments
of puppies they help ensure that you will get a healthy addition
to your family who will provide you with companionship and joy
for many, many years.
Dr. Andrew Jones, DVM
Dr. Andrew Jones
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