4 T Horses Leathal White Overo Information

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4 T Horses Ranch

Leathal White Overo FAQs

A DNA-based test to identify carriers of the lethal white overo (LWO) gene

Horse breeding programs specializing in overo have particular challenges compared with programs for other white patterns such as tobiano. Not only is there the possibility of producing a solid dark foal without the overo pattern but there is also the risk of producing an all-white foal that dies of complications from intestinal tract abnormalities (aganglionosis). As far as we are aware, overo horses themselves have no specific health risks. While breeding evidence shows that some overos are heterozygous for a gene that is lethal in the homozygous condition, it has not been easy to identify which horses have the overo gene that is associated with the lethal condition (LWO). Occasionally even solid-colored horses without obvious body spotting patterns have been reported to sire or produce lethal white foals. Clearly the spotting pattern classified as overo is phenotypically and genetically heterogeneous. Over the last thirty years, breeders have sought advice to recognize those overos at risk to produce lethal white foals, but up to now no reliable trait identification system has been found.

Fortunately, recent research has identified the gene that produces the overo lethal and a DNA-based diagnostic test is now available to identify breeding horses at risk to produce lethal white foals. Breeders can use the test not only to avoid producing lethal white foals but also to identify new pedigree sources of the overo gene that may be useful in their particular programs. The gene appears to be associated with those overos often characterized as "frame overos" in Paints and Thoroughbreds, but is also present in some tobiano/overos, in some solid-colored (breeding stock Paint) offspring from overo matings and in some tobianos and Quarter Horses without obvious evidence of the overo pattern. The gene has also been identified in an overo Miniature Horse.

Genetic studies based on the new DNA diagnostic test for the LWO gene confirm the proposal that the lethal white overo is due to an offspring having two copies of a gene that in single copy usually produces the attractive color pattern known as overo. Using the letter "O" to symbolize the DNA sequence of the lethal white overo gene and "N" for the sequence of the non-overo, then the lethal white foals can be symbolized as OO, their overo parents as NO and non-overos as NN. When two NO overos are bred together, then it is predicted that 25% of the offspring will be lethal white (OO). The living offspring will be either overo (NO) or solid ("breeding stock") (NN). By avoiding the breeding together of stallions and mares that have a single copy of the lethal white overo gene (NO), it is possible to produce foals with the overo pattern without the possibility of producing lethal whites. From breedings between a solid (NN) and an overo (NO), solid and overo foals are expected in approximately equal numbers. These possibilities are shown below in checkerboard diagrams familiar to students of Mendelian genetics:

Breedings between LWO overos (NO x NO):

�� |�� N�� |�� O�� |

N |�� NN�� |�� NO�� |

�� |25% Solid|25% Overo|

O |�� NO�� |�� OO�� |

�� |25% Overo|25% Leathal|

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

�� |�� N�� |�� O� ��|

N | ��NN� ��|� ��NO� �|

__|25% Solid_|25% Overo|

N | ��NN� ��| ��NO� ��|

�� |25% Overo|25% Overo |

No possibility of lethal white foals

The diagnostic test for overo uses a process known as ASPCR (allele specific PCR). 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. We know of no other mutations that are associated with lethal white overo horses. However, owners requesting the diagnostic test should be certain to understand that there is the rare possibility that two NN horses could have a lethal white foal due to the sire and dam having in common mutation at another site than the one detected by this test.

Q: What is the overo spotting pattern? A: On the one hand, overo can be loosely defined as any white spotting pattern that is not conventional markings, not tobiano and not appaloosa. Refer to the rules of the Paint Horse Association for a more descriptive definition. More than one gene is likely to be involved in the overo phenotype. As the genetic complexity of the overo pattern has become more obvious, the descriptive terms frame overo, sabino and splash white have been more frequently used to attempt to define the variations. The spotting pattern in horses with the LWO gene generally corresponds to the frame overo pattern, with white markings on the head and sides of the body, not necessarily on the legs, the belly or the dorsal midline.

Q: I have a frame overo Paint stallion that has sired three lethal white foals. Should I have him tested? A: You have genetic evidence in the form of progeny testing to define his genetic status. He is an NO. You will want to advise mare owners that to avoid producing lethal white foals, they should not breed mares to him that have produced lethal whites or mares that test to be NO. Quarter Horse and Thoroughbred mares are unlikely to have the LWO gene, but if there is any doubt, they can be tested with the overo test to make certain.

Q: I have a two-year old overo Paint colt that we plan to stand this year. His overo sire has sired lethal whites. Will my horse also have this gene? A: Probably yes, but now that there is a test, you can test him and you will know with certainty.

Q: My neighbor bred her overo mare to a tobiano/overo stallion and got an all-white filly that is now six years old. The foal never had symptoms of colic and has become a fine riding horse. The owner wants to breed her to our overo stallion that has sired lethal white foals. Should we encourage her to have this mare tested?

A: Your neighbor has a horse that is an example of a genetic combination that produces a viable white horse. We have tested two such horses with the overo test and both tested to be NN for the LWO gene. Until we have evidence that there is no possibility for white horses to be NO, it would be wise to have them tested prior to breeding to known NO stallions.
NOTE: Cremellos and Perlinos are often mistaken for a "lethal white" that somehow survived. For more information on that topic, click HERE to read up on it.

Q: Your studies have identified a Miniature Horse with the mutation. I have never heard of a lethal white in Minis. A: In Minis the OO combination may cause early embryonic, not neonatal loss. This situation could also occur in Paints, although at present no evidence is available to support this explanation.

Q: I raise parti-colored Arabian horses that may have white markings on the ventral mid-line, white over the knees and hocks and white spots in the girth area. My neighbor says that if they were Paints, they would be called overos. Should I have them tested? A: In our preliminary studies, we tested several parti-colored Arabians and none had the LWO gene. At this point, we have no evidence to suggest that the LWO gene is present in Arabians.

Q: If all OO horses die, does that mean that there is no living overo that is true-breeding for overo? A: As far as we know, there are no living homozygotes for this overo gene, but we do not know if that is true for the other types of overo genes. Stallions that are compound heterozygotes for more than one spotting gene (such as overo and tobiano, or two different overo genes) can sire a high percentage of spotted foals (but not 100%).

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		4 T Horses Ranch
		Leathal White Overo FAQs
		A DNA-based test to identify carriers of the lethal white overo (LWO) gene Horse breeding programs specializing in overo have particular challenges compared with programs for other white patterns such as tobiano. Not only is there the possibility of producing a solid dark foal without the overo pattern but there is also the risk of producing an all-white foal that dies of complications from intestinal tract abnormalities (aganglionosis). As far as we are aware, overo horses themselves have no specific health risks. While breeding evidence shows that some overos are heterozygous for a gene that is lethal in the homozygous condition, it has not been easy to identify which horses have the overo gene that is associated with the lethal condition (LWO). Occasionally even solid-colored horses without obvious body spotting patterns have been reported to sire or produce lethal white foals. Clearly the spotting pattern classified as overo is phenotypically and genetically heterogeneous. Over the last thirty years, breeders have sought advice to recognize those overos at risk to produce lethal white foals, but up to now no reliable trait identification system has been found. Fortunately, recent research has identified the gene that produces the overo lethal and a DNA-based diagnostic test is now available to identify breeding horses at risk to produce lethal white foals. Breeders can use the test not only to avoid producing lethal white foals but also to identify new pedigree sources of the overo gene that may be useful in their particular programs. The gene appears to be associated with those overos often characterized as "frame overos" in Paints and Thoroughbreds, but is also present in some tobiano/overos, in some solid-colored (breeding stock Paint) offspring from overo matings and in some tobianos and Quarter Horses without obvious evidence of the overo pattern. The gene has also been identified in an overo Miniature Horse. Genetic studies based on the new DNA diagnostic test for the LWO gene confirm the proposal that the lethal white overo is due to an offspring having two copies of a gene that in single copy usually produces the attractive color pattern known as overo. Using the letter "O" to symbolize the DNA sequence of the lethal white overo gene and "N" for the sequence of the non-overo, then the lethal white foals can be symbolized as OO, their overo parents as NO and non-overos as NN. When two NO overos are bred together, then it is predicted that 25% of the offspring will be lethal white (OO). The living offspring will be either overo (NO) or solid ("breeding stock") (NN). By avoiding the breeding together of stallions and mares that have a single copy of the lethal white overo gene (NO), it is possible to produce foals with the overo pattern without the possibility of producing lethal whites. From breedings between a solid (NN) and an overo (NO), solid and overo foals are expected in approximately equal numbers. These possibilities are shown below in checkerboard diagrams familiar to students of Mendelian genetics: Breedings between LWO overos (NO x NO): �� \|�� N�� \|�� O�� \| N \|�� NN�� \|�� NO�� \| �� \|25% Solid\|25% Overo\| O \|�� NO�� \|�� OO�� \| �� \|25% Overo\|25% Leathal\| Breedings between LWO overo and solid (NO x NN): N O �� \|�� N�� \|�� O� ��\| N \| ��NN� ��\|� ��NO� �\| __\|25% Solid_\|25% Overo\| N \| ��NN� ��\| ��NO� ��\| �� \|25% Overo\|25% Overo \| No possibility of lethal white foals The diagnostic test for overo uses a process known as ASPCR (allele specific PCR). 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. We know of no other mutations that are associated with lethal white overo horses. However, owners requesting the diagnostic test should be certain to understand that there is the rare possibility that two NN horses could have a lethal white foal due to the sire and dam having in common mutation at another site than the one detected by this test. Q: What is the overo spotting pattern? A: On the one hand, overo can be loosely defined as any white spotting pattern that is not conventional markings, not tobiano and not appaloosa. Refer to the rules of the Paint Horse Association for a more descriptive definition. More than one gene is likely to be involved in the overo phenotype. As the genetic complexity of the overo pattern has become more obvious, the descriptive terms frame overo, sabino and splash white have been more frequently used to attempt to define the variations. The spotting pattern in horses with the LWO gene generally corresponds to the frame overo pattern, with white markings on the head and sides of the body, not necessarily on the legs, the belly or the dorsal midline. Q: I have a frame overo Paint stallion that has sired three lethal white foals. Should I have him tested? A: You have genetic evidence in the form of progeny testing to define his genetic status. He is an NO. You will want to advise mare owners that to avoid producing lethal white foals, they should not breed mares to him that have produced lethal whites or mares that test to be NO. Quarter Horse and Thoroughbred mares are unlikely to have the LWO gene, but if there is any doubt, they can be tested with the overo test to make certain. Q: I have a two-year old overo Paint colt that we plan to stand this year. His overo sire has sired lethal whites. Will my horse also have this gene? A: Probably yes, but now that there is a test, you can test him and you will know with certainty. Q: My neighbor bred her overo mare to a tobiano/overo stallion and got an all-white filly that is now six years old. The foal never had symptoms of colic and has become a fine riding horse. The owner wants to breed her to our overo stallion that has sired lethal white foals. Should we encourage her to have this mare tested? A: Your neighbor has a horse that is an example of a genetic combination that produces a viable white horse. We have tested two such horses with the overo test and both tested to be NN for the LWO gene. Until we have evidence that there is no possibility for white horses to be NO, it would be wise to have them tested prior to breeding to known NO stallions. NOTE: Cremellos and Perlinos are often mistaken for a "lethal white" that somehow survived. For more information on that topic, click HERE to read up on it. Q: Your studies have identified a Miniature Horse with the mutation. I have never heard of a lethal white in Minis. A: In Minis the OO combination may cause early embryonic, not neonatal loss. This situation could also occur in Paints, although at present no evidence is available to support this explanation. Q: I raise parti-colored Arabian horses that may have white markings on the ventral mid-line, white over the knees and hocks and white spots in the girth area. My neighbor says that if they were Paints, they would be called overos. Should I have them tested? A: In our preliminary studies, we tested several parti-colored Arabians and none had the LWO gene. At this point, we have no evidence to suggest that the LWO gene is present in Arabians. Q: If all OO horses die, does that mean that there is no living overo that is true-breeding for overo? A: As far as we know, there are no living homozygotes for this overo gene, but we do not know if that is true for the other types of overo genes. Stallions that are compound heterozygotes for more than one spotting gene (such as overo and tobiano, or two different overo genes) can sire a high percentage of spotted foals (but not 100%).