Lethal White Overo (LWO) syndrome, also called overo lethal white syndrome (OLWS), occurs when
a horse is homozygous (OO) for
the frame overo gene. This genetic disorder causes the intestinal
system not to develop properly (involving aganglionosis of the bowel).
The foal will die within the first 72 hours after birth when its
first meals cannot be digested properly. The lethal white foal will
be born almost pure white.
This genetic abnormality is caused by a dinucleotide
TC-->AG mutation, which changes isoleucine to lysine of the EDNRB
protein. Horses that do not have LWO syndrome can still be carriers
of the LWO gene. When they are carriers of this gene, they are said
to be heterozygous (nO) for
the LWO gene and may pass it on to offspring. The heterozygous LWO
gene in a horse occurs when the diploid (one copy from mother and
one from father) of the LWO gene contains one frame overo copy and
one non-frame overo copy and is often referred to as positive for
Courtesy of Cara and Jesse Daniels
Since frame overo is a desirable quality and
requires one frame overo copy, proper mating must be done to avoid
possible loss due to lethal white overo while still achieving a
high probability for the frame overo pattern. The way to avoid this
problem is to avoid breeding frame overo to frame overo. Identification
of frame overo can only be definitively done by genetic testing
since the frame overo pattern is not always expressed and can be
masked by other genetic traits. The gene has been associated with
Paints, Thoroughbreds, Quarter Horses as well as Miniature Horses.
A universal method of describing the genetic
testing results has been adopted. The letter "O"
symbolizes the DNA mutation of the lethal white foal syndrome (LWFS)
overo gene and "n" for the normal, non-overo allele.
Therefore, the lethal white foals or Homozygous positive for the
mutation are characterized as OO.
Heterozygous horses or overos are identified as nO
and Homozygous negative or non-overos as nn.
This chart shows the possibilities of offspring when mating two
- one overo/one non-overo - probable overo pattern
- both non-overo - no overo pattern
OO - lethal white overo - horse
will not survive
Breeders breeding two overo horses (heterozygous
nO) can expect a 50% chance of producing an overo foal, a
25% chance of producing a lethal white foal OO
and a 25% chance of producing a non-overo foal.
Breeders breeding a frame overo horse (heterozygous nO)
with a non-overo horse (homozygous negative nn)
can expect a 50% chance of producing an overo foal nO
and no chance of producing a lethal white foal OO.
There is a rare possibility of there being another
mutation within this gene that causes lethal white overo in addition
to the one we test for. Even when a horse has been tested nn,
it still has the rare possibility of carrying this other mutation
that causes lethal white overo.
White Overo Testing
Animal Genetics offers
DNA testing and detection of the gene mutation responsible for Lethal
White Overo syndrome.
US per sample.
Collect sample by pulling (not cutting) 20-30
mane or tail hairs with roots attached. It is important that you
pull the hairs and confirm that the actual root of the hair is being
collected. The root contains the genetic material of your horse
that is needed for DNA testing. Therefore, cut hairs do not provide
an adequate sample of your horse. Place the collected hairs of each
horse in a separate zip-lock bag labeling the bags accordingly with
the horses name or identification number. Download and complete
a submission form for each sample and send along with
payment to Animal Genetics for testing.
Results are given using the following symbolic
positive for the lethal white overo gene. LETHAL condition.
|Both non-overo and
overo alleles detected. Horse tested heterozygous for LWO and
can be considered an overo carrier.
for the frame overo gene. Horse can be considered a non-carrier.