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      Miller's Miniature Horses Presents...

     I Carry The
        White Lethal Overo Gene In My Herd!

I Carry The White Lethal Overo Gene In My Herd!
By: Carolyn Miller

          Why would anyone want to make that statement? Well according to the research that has been done by Ann T Bowling all the frame overo horses in her study were heterozygous for the mutation that is associated with the white lethal foal syndrome, explaining why lethal white foals are produced predominantly by frame overos. The data collected and studied supported the facts that the Lethal White Foal syndrome was caused by homozygosity for a mutation in the EDNRB gene of horses, and that the overos heterozygous for the mutation display the frame overo phenotype. (In lay terms, the frame overo contains one copy of the lethal white gene from one parent only, and the lethal foal contains two, one from each parent.)

How can solids have lethal white foals then? It is quite possible that the solid parent is genetically an overo (frame) and was unrecognized as such. Again, it is extremely helpful to know exactly what pattern you are breeding, and with genetic testing today there should be no doubts.

        I personally started working with overos over eight years ago and have been studying and breeding with them since then. About three years ago the product of Calico, a loud frame overo stallion bred to Velvet a solid bay mare of mine produced a son, Chaps, who was a solid bay, with two bright blue eyes, and roan hairs in his flanks and on his face. After reading the research that Ann Bowling had done with the white lethal gene at Cal Davis I decided knowing Chaps background it would be worth the test to designate whether he could carry the white lethal gene which would then make him capable of producing the much sought after frame overo pattern.

Van Lo's Overo Kid "Calico" owned by Paul & Lesley Cossey was breed to Velvet a bay mare to produce "Chaps" the blue eyed DNA'd overo. Calico is double reg. A&R and he is '99 Nat'l A div. Color Champion, 2000 PTHA World Res. Ch. Color A div.

To my knowledge and to date this testing process is only connected to the frame pattern and the other overo patterns known as splashed white or sabino cannot be tested for and are not connected with this white lethal gene unless they are additives with the frame overo gene present in the same animal. It was no surprise to me that Chaps came back as a designated NO, which means he got one O-overo gene and one N-non overo gene from his parents. Again, this meant he carried the frame overo gene as we knew what classification his parents were and it also meant as this is considered a dominant it would only take the breeding of Chaps to any other horse of any other color to produce the popular frame overo pattern. If we breed him to another carrier of the "O" gene then the research data suggested that the offspring would be 25% lethal, 50% overo and 25% solid. Keep in mind that you can have a genetic overo that phenotypically may appear to be solid, as did Chaps.

        It helps to know what genotype means (what they carry in their genes) and what phenotype means (what you actually see.) What you see does not necessarily mean that is what that animal will produce. Certain colors can actually mask genes (cover them over). Also EDNRB is the endothelin B receptor gene sequence which is associated with the Lethal White Foal Syndrome, which is what our most recent testing is based on, however there is a "rare" possibility that two NN (non-overo) horses could produce a lethal white foal due to the sire and dam having a common mutation at another site totally different than one being tested for at present found in the frame overo. The white lethal foal must receive a copy from each of its parents in our recent overo testing and the code would appear "OO" with both parents having been genetic overos and the foal would then have been a homozygous overo that would soon die after birth due to the intestinal abnormality similar to the human counterpart "Hirschsprung Disease". You also need to know that tobiano marked horses included carriers, indicating that tobiano is epistatic (covers over, or masks) the frame overo.

        To give you another example of a solid appearing foal that tested positive for the white lethal gene with the overo background let us look at my filly "Ginger". A loud medicine Hat Tovero sired Ginger, who himself was out of a loud frame overo daughter of the reknowned famous frame over "Sample". Ginger came back NO, which means she got the N from her Dam with no known overo background and Ginger got the O from her sire the Medicine Hat Tovero with frame background. The testing allows me to make choices as to whether or not I personally want to risk the breeding of her to another frame overo and potentially produce 25% white lethals, with an outcome of 50% potential frame overo offspring. The genetic testing also allows a purchaser the option of testing for the LWO (Lethal white overo) to find out whether or not that horse will be capable of producing the spectacular pattern known as the frame overo If you see a horse as the product from one or two frames that is a solid and it comes back NN when you tested then you can bet it will not produce a frame patterned foal for you unless you breed that horse to another frame patterned horse. The new testing allows the breeder wider latitude in what we can do with our breeding program and do it the way we would like.

No, we cannot do testing for "overos" as such, but we can test for the altered sequence in the EDNRB gene associated with a particular pattern known as the "Frame Overo".

        I for one, have horses that test positive for the one copy of LWO both displaying the colorful frame overo pattern and showing only small signs indicating they have the pattern. I have tested all my horses so I "know" what I am breeding with and can project my potential success at breeding what I want. I also can give my clientele the option of knowing that their horses either are capable or non capable of producing the same. Again, there is no known test available to give you data for the Sabino or Splashed (belly) at this time that I am aware of.

        For test results I would highly recommend going to the fountainhead lab in the business at Cal. Davis where the testing for the LWO was pioneered. For topics related to genetics you may visit the Univ. of Calif. Davis at their website: I further recommend that anyone becoming involved with the breeding of the overo frame pattern purchase a copy of Ann T. Bowling's book. "Horse Genetics" which can be procured through UCDavis bookstore. Further reading appropriate to this is Barbara Naviaux's book, "Miniature Horses" Their care, Breeding and Coat colors. An article was published through the Miniature Horse World in Aug/Sep of 1998 on p 76 and finally a good description is given with the return of your test results from the labs at UC Davis which can be contacted by calling: 530-752-6206 and requesting forms for submission of samples that can be done via hair.

        Many thanks go to Barbara Naviaux, a friend who confirmed through my phone conversation with her that my understanding of this work was in fact correct. I believe that in order to produce a frame overo, you must have one copy at least of the LWO in one of the parents, and that to produce a white lethal you would have to have two frame overo parents each giving their LWO gene to their offspring. I have always valued Barbara's thoughts and we have kept in touch over the years sharing genetics and historical data with each other.

        Now that we have an understanding of what the white lethal is and does there should be no hesitance in advertising that you carry within your herd the white lethal gene as it tells onlookers of your advertising that you possess the capabilities of producing the beautiful and much sought after "Frame Overo" pattern.

        It also allows your followers to choose what pattern they may want to breed with dependant on their own knowledge of what lethal white does and how the percentages work.

        Per usual my work is subject to error or omissions and is based on the following in print information with regards to the lethal white overo and it's connection to the frame overo pattern.

Reprinted from the Minature Horse Voice October 2000 Vol 7, No. 12 All Rights Reserved

Solid color filly. "Ginger" by the Miller's stallion DNA'd for white lethal. Stallion was a Bonneted Tovero bred to a solid mare to produce Ginger, herself DNA'd at Cal Davis to hold NO, one non overo and one overo designating the white lethal. Ginger's sire went back to a long line of frame overos
VanLo's Cookie N Cream owned by Paul & Lesley Cossey also sired by Calico is a loud frame overo. She is the '98 Nat'l color A mare, '99 Nat'l. color B mare and 2000 PTHA World Champ, Halter and color B division. She is also half sibling to Chaps.
Miller's Impressive Overo, a black and white Miller bred frame overo colt the product of a frame overo and solid color cross breeding.
"Sample" with Mrs. Lois Myler of Van Lo's Rainbow Ranch. Sample was the foundation for the Myler frame overos and is pictured here at over 30 years old. He is the Great Grand Sire to "Ginger" noted in the article as a solid filly DNA'd for the one copy of the LWO noting overo.

Photos and Content Copyright © 2001 Millers Miniature Horses
Web design by Rain Silverhawk

Graphics Copyright © 2001 Sam Silverhawk. All Rights Reserved

Disclaimer: (Content on this site subject to errors and or omissions.)


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Carolyn Miller writes The Journal's column for Area III Southern Sunshine. She has had numerous feature articles written in various horse publications pertaining to the equine world including history, medical and genetic related topics. She and her husband Jerry own and operate Miller's Miniatures in Abbeville, S.C. where they care for a select herd of Miniature Horses. She is also noted for her photographs related to equine. Presently she and her husband are breeding for and studying the "frame overo" pattern and has also brought together the sabino and belly splashed patterns to produce multiple patterned horses allowing for greater percentages of the sought after colors.

-Horse Genetics by Ann T. Bowling
-Miniature Horses by Barbara Naviaux
Understanding your DNA test results,
by Barabara Naviaux MHW 12/99
-DNA based Lethal White Overo Gene Test
Now Available by Vet lab @ UC Davis
-Coat Color Genotype Testing by Barbara Naviaux the Journal 6/99