Over the years as a breeder I have encountered many people who are confused regarding the breeding of horses of the overo colour pattern and of the chances of breeding a foal with the Lethal White Syndrome.
In days gone by Paint horses used to be categorised as either Tobiano or Overo for registration purposes.
This is most likely at the basis of most of the confusion.
To date there are four colour patterns that have been recognised amongst Paint and Pinto horses. All patterns can be expressed from a minimal to extensive form.
TOBIANO
SABINO
OVERO
SPLASHED WHITE (Another source of confusion as some countries call loud overos splashed overo because of the splash like effect of the pattern, which is different from a true splashed white.) A true splashed white will be white on the bottom half of the body as if the horse had been picked up and dipped in white paint and can be deaf, the face is bald and it has two blue eyes. This is not a pattern that breeders strive for in Australia, because of the possiblility of deafness.
Things can become more confusing as people start to mix one colour pattern with the other and we get horses that are a mixture of two or more patterns and are termed,
SABERO (sabino + overo), TOVERO OR TOBERO (tobiano + overo) , TOBINO (tobiano + sabino) and TOVINO (tobiano + overo + sabino).
For registration purposes, anything not an obvious Tobiano would be most likely have been categorised as an Overo. This has now changed.
Things become further confused because the SABINO and OVERO patterns at times possess many similarities.
There are also horses known as MEDICINE HATS. These are nearly all white, often with only the ears remaining with colour, or sometimes completely white with speckles over the skin...these horses are generally a result of sabino x overo crosses or sometimes tobiano x overo.
Getting back to OVERO.
For explanation purposes I Shall call Overo O on this Page and Non Overo N as I cannot display the correct symbols with my keyboard.
Overo is usually O with a smaller o above and non overo is usually an O with a + sign above.
Very simply, all foals are made up of two sets of genes, one strand received from the Sire and one strand received from the Dam.
The overo foal O/N, has received one overo gene,O, from one parent and one non overo gene, N, from the other parent.
We now know that when a foal receives an Overo gene from each parent, O/O (Homozygous overo) the foal will be what is termed a lethal white. The foal is born all white (occasionally with slight spotting) with light blue eyes. They have an incomplete digestive system and will not survive more than a few hours after birth. They will exhibit pain and discomfort and should be euthanized accordingly. (To date no attempt to surgically correct the defect has been successful)
This is why we have never seen a horse which claims to be Homozygous Overo.
We now have a test to positively identify horses carrying the overo gene...it is called an OVERO LETHAL WHITE (OLW) or OVERO LETHAL WHITE SYNDROME (OLWS) test.
To my mind it should be simply called an OVERO test as many people are under the false assumption that there is a LETHAL GENE and that you can have an OVERO that is negative for LETHAL. This is simply not so. Every true overo horse has the propensity to produce a lethal foal if bred to another true overo.
If a horse is thought to be an OVERO and it has a negative OLW test it is most likely a SABINO not a true OVERO at all.
This is why it is not adviseable to breed two horses exhibiting the overo colour pattern together. Some people will argue that they have done it and that they got away with it but if both horses are truly positive for overo, they are really playing a game of russian roulette.
The resulting foal may inherit O from the sire and N from the dam, or similarly it might inherit N from the sire and O from the dam and the breeder will get away with it having a O/N (overo) foal each time. The breeder could also have a foal which is solid (not coloured) because it has inherited N from each parent resulting in a N/N foal but they are still taking the risk of the foal inheriting O from each parent and getting a Lethal White O/O.
The best option when the desired outcome is a healthy foal is to breed a horse which is positive for overo to a horse which is negative for overo. The statistics work out at 50 % chance of overo and 50% chance of a non overo.
Whilst an overo to overo mating has 50% chance of overo, 25% chance of a non overo and 25% chance of a Lethal White Foal.
If you are looking to breed your horse and you are uncertain if your horse carries the overo gene it would be wise to have the test done.
We would recommend that any horse of overo parentage which displays white somewhere on it's body, even if it's just a white sock should be tested so that the owner knows for certain.