The overo pattern can be so minimal as to be a star or so extreme as to cover the whole horse.

EVERY overo horse carries the Lethal White Overo gene (Keep reading this page for more information on the LWO gene).

• Horses with this pattern will commonly have dark legs. 
• The head of an overo horse usually has a white marking that may range in size from a very small star to a large blaze that may be an odd shape. 
• The white areas have clean edges but are usually more jagged than is seen with the Tobiano pattern. 
• The white on an overo horse tends to have more of a horizontal arrangement rather than the vertical arrangement seen with the Tobiano pattern.
• Full white heads with colour only on the ears and sometimes surrounding the ears is called a "medicine hat"
• Commonly a horse with a white facial marking will have some pigment on their lips (black areas)
• Blue eyes are common on overo horses, even if the eye is surrounded by colour rather than white.

The overo pattern is a caused by a dominant gene. It has been proven with laboratory research that horses that appear to be solid in colour CAN carry the overo gene if one of its parents is overo - the pattern DOES NOT have to be expressed to be present.  

For this reason, a horse that does not look overo may in fact carry the lethal gene!

The overo gene in homozygous form is lethal and produces foals that have OLWS (Overo Lethal White Syndrome). In other words -  any foal that gets TWO overo genes (one from each parent) will be a lethal white foal...
These foals if carried to full term, are born totally white, although some may have a tiny spot or two of colour. These foals have an incomplete lower colon and because of this the foals cannot defecate and die of colic shortly after birth. It is recommended that these foals be humanly euthanized, but it is very important to know for sure that a white foal is in fact a Lethal White because white foals can be born that are totally healthy and normal (such as extreme sabinos or very loud overos with one overo gene). 

NOTE: Sabino and splash white horses do not carry the lethal white gene and can be safely crossed with overos with no concerns of lethal white foals.  This of course does not include horses that carry overo AND splash or sabino... this horse would still carry lethal white.

There is a laboratory test that can be done to determine if a horse is a OLWS carrier. For more info, click here.  

I urge anyone with an overo bred horse to have this horse tested before breeding it to another overo bred horse... although the chance of a lethal white foal from 2 overo parents is only 25%, breeding responsibly will prevent ANY lethal white foals.  Lethal white foals have very short painful lives.

Read MORE ABOUT LETHAL WHITE FOAL SYNDROME

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