Lethal White Overo Syndrome
Different from Double Dilutes

All though the Lethal White Overo (LWO) syndrome resembles the characteristics of horses carring two copies of the creme dilution gene (Cr) they should not be confused with one another.  Both foals are born with pink/salmon skin, white/creme hair coat and blue eyes.  The difference being that LWO foals die within 48 hours of birth due to complications from intestinal tract abnormalities (aganglionosis) , where as double dilutes are perfectly normal and healthy foals.

One must evaluate a foal closely that has these characteristics and the parents color background.  There may be a  possibility of a cremello or perlino if two diluted colors are crossed.  For instance in the event that two overo palominos were crossed there is a possibility of a cremello without carrying the LWO gene.  There is a difference!

The differences between Lethal Whites & Double Dilutes:

 The difference is that the LWO Syndrome is caused by the homozyogous result of a pattern gene and the double dilute is caused by the homozygous result of a dilution gene. Both resulting in chance of producing an all-white foal. However there is a major difference between the two is that the LWO will die of complications from intestinal tract abnormalities within 48 hours of birth.   The double dilute foal on the other had will mature and live a normal life.  

It must also must be noted that if both parents carry the LWO gene and the dilution (Cr) gene, there is a possibility of the offspring being homozygous for both conditions.  It is important to not put down the resulting foal before knowing for sure they are O/O.  If the foal survives it is a double dilute and not a lethal, however it can also be heterozygous for the LWO gene.  Meaning that they only carry one gene and can pass it to their offspring.  

Occasionally even solid-colored horses without obvious body spotting patterns have been reported to sire or produce lethal white foals.  Tobianos and tovero color patterns have also been associated with LWO.

This is the reason genetic testing is an important precaution when breeding paints. Research has identified the gene that produces the lethal white overo and a DNA-based diagnostic test is now available to identify breeding horses at risk  The gene appears to be associated with  "frame overos", but is also present in some tobiano, toveros, and even some solid-colored (breeding stock Paint) offspring from overo matings.   It has also been reported Quarter Horses without obvious evidence of the overo pattern. 

Studies have indicated that the lethal white overo is due to offspring carring two copies of the LWO gene, one gene passed from each parent.  Therefore both the mare and the stallion must carry the LWO gene.  When a single copy is passed the popular overo pattern is produced.

There is a  test for overo!  The test can be performed from either blood or hair samples with roots (hair preferred). The test identifies a specific mutation site in the DNA sequence that has been shown to be associated with lethal white overo foals.

There is no DNA-based diagnostic test available for the Cr gene as of yet.

For more information:

http://www.vgl.ucdavis.edu/horse/lethalwo.htm
UC Davis

 Breedings between LWO  overos 
(NO x NO):

        Breedings between LWO overo and solid (NO x NN):

  No possibility of lethal white foals.

Important to Note

Tobianos & Toveros
reported w/ LWO

Genetic Testing
Important

Both parents
Carry LWO gene
to produce lethal

Test Available!

No test for Cr gene