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What? No way! Impossible! How could that be? Are YOU asking any of
these same questions? I would hope so! If you are, then you obviously have a little
interest in finding out about this, and what to do to avoid such a terrible event.
Have any of you had a seemingly normal tobiano foal die for no apparent reason?
Well, read on....this piece could show you one possible reason for it.
Let me first begin this with a few basic genetics lessons to enable you to better understand a few of the genes involved, and their inheritance. Breeders must first be aware fo the existence of the two patterns Paint Horses can pass on to their offspring. Tobiano and overo are two separate patterns and are controlled by separate genes. In and of themselves, the genes have nothing to with each other. In certain matings, however, both patterns can be seen on the body fo the foal. This combination is called "tovero", but tovero is no more a gene than is Morab (Morgan X Arabian) a breed. Basically, both are just crosses between the two entities involved. Tobiano is a dominant gene, and every foal who inherits the gene from at least one parent will be a tobiano foal (one parent must be tobiano in order to be able to pass on pattern). The tobiano does not mask other patterns such as overo, or appaloosa; if it did, we would not have tovero or tobi/appaloosa horses in our midst. Every horse has two genes (that we know of to date) capable of carrying tobiano pattern with them. The genes either do, or they don't. Two tobianos, for example, who create a non-tobiano foal together, have created a foal who has no copies of the pattern in its genotype, and without tobiano in the genotype (genes), there can be no tobiano passed on to manifest itself in the phenotype (outward expression [on the body]). A non-tobiano gene is referred to by breeders as a "solid" gene. Quarter Horses have two solid genes. Overos have two solid genes. Arabians have two solid genes, and so on and so forth. On paper, the Tobiano gene is labeled as "T". The non-tobiano gene is labeled as "t". A Quarter Horse, then, is labeled "t t" on paper--one "t" for each of the two non-tobiano genes it has. An Arabian and an Appaloosa would be labeled the same way....they are both non-tobiano. Each horse receives one gene from each parent which results in the foal having his pair. A tobiano who has one tobiano parent and one non-tobiano parent is labeled T t. The "T" for the patterned gene it got from its tobi parent, and t for the solid gene it got from its other parent--be it Arab, Appy, Quarter, Overo, or breeding stock Paint, etc. A homozygous tobiano, TT, received one colored gene from each parent, and in order for a horse to be homozygous, it must have two tobi parents. (Have any of you heard about certain homozygous tobianos having their pedigrees pulled? If so, it is because these horses have been documented with either Quarter, overo, or some other such type of horse...and this type of cross is impossible to create a TT horse from.) Two TT horses always create TT foals. Two t t horses always create t t foals. Two Tt horses can create a tt, Tt, or TT foal. Any cross involving a TT horse will always result in a colored foal. A TT crossed with a Tt will always result in a colored foal, but the foal could be either TT or Tt. Separating the genes in the lab by markers is a whole other issue and not part of this; so, I will not go into the scavenger and vitamin D-binding proteins which are vital for doing this to prove our horses as homozygous, or heterozygous; informative, or uninformative. Tt horses are called single-gened, or heterozygous. TT horses are called double-gened, or homozygous. A single-gened horse can have two tobiano parents, but both parents cannot be TT. If a TT horse parents a foal who, "in the registry" is papered a breeding stock, then the pedigree on the TT parent is false, or the foal is misregistered. There are tobianos who don't have big enough spots, or spots in the places humans think appropriate, to be qualified as tobianos or regular registry, if you will. However, Mother Nature puts spots in the places she so chooses, and in the size and quantity she so chooses...regardless of what we agree or don't agree with. This "breeding stock" foal, of course, will go on to create tobiano foals with non-tobiano mates...and genetically and scientifically this is an impossible feat. So who is wrong in the registration of these foals, hmmmmmm? Roughly 50% of the time, these "solid" horses will parent tobianos...but the laws of averages may give them more or less than that. In either case, if this foal creates tobi foals with non-tobi mates, then the proof is in the puddin' that the horse really is tobiano...no matter what the breed registry is willing to admit. Mother Nature simply does not provide a blueprint for the genes to follow to put spots on properly to satisfy our silly rules. I believe that "solid" foals parented by proven homozygous horses may as well be granted regular registry from the get go....that way, the associations won't have to answer for acknowledging and accepting the facts that TT horses exist, but then on the other hand explain with validation why they try to call some of the foals solids!! That is impossible. To not consult Mother Nature when need be...I feel we only serve to show our ignorance and outright defiance of Her. Many people think the overo gene is recessive in the sense that it takes pattern from two overo parents to pattern the foal. This is just not the case, either. If it were, then how does one explain the many overo foals resulting from overo to Arabian crosses? I offer that the overo is a dominant-recessive. It is a recessive gene in that it doesn't necessarily put pattern onto every foal, but when it does decide to color the foal, it can do so without contribution from the other parent. I have bred overo to Arabian and created colored foals. You must have tobiano on the body to get a tobiano; but, you do not need overo on the body to get overo...which explains why breeding stock overos sometimes have colored foals seemingly out of the blue on their own. Blame is laid upon the overo gene as the culprit for the lethal white. In our Paints and Pintos, this is pretty much the case. However, with all the years of breeding and crossing the two patterns, tobianos and toveros now carry the lethal, too. Tobianos normally are not carriers. There is a test now for the lethal and is reported to be highly accurate. Admittedly, I have not studied much about it yet as I concentrate on quality tobiano foals...lethal doesn't enter into my picture much. But this is on my list of 'to do' things...! A horse who carries the lethal has only one copy of the deadly gene. If it had two, it would be dead. Lethal in the homozygous state is a dead horse. Carrying does not physically hurt the horse who carries it, but when bred to another carrier, there is a 25% chance of a signed death warrant for the resulting foal. The parents could each pass on lethal, only one of them, or neither of them. It is said that all overos carry the lethal, but I just cannot accept that fact yet. Ok, on to lethal TT horses now that this little bit of "Genetics 102" is out of the way. I'm going to present a scenario for you in which you will be able to follow just how lethal homozygous foals can be created. With this and the ability to test overo-bred horses, it should be easy for anyone to prevent this from happening. Mr. Jones has a nice overo stallion whom he bred to his own tobiano mare. The following spring, she laid down and produced a fine tovero filly, whom the man named Beauty---she received pattern from both parents. Mr. Miller, a neighbor down the road a piece, bred his overo mare to a neighboring tobiano stallion in the next county....the same season Mr. Jones bred his own mare. The foal in Mr. Miller's cross resulted in a strapping tovero colt. He, too, received pattern from both parents. Mr. Miller was proud, and called his colt Hotstuff. Both men had bred their overos together once before and this was when they found out that both horses carried the lethal white....Mr. Jones' overo stud, and Mr. Miller's overo mare...the foal was white and dead. Thus, the crossing of their horses elsewhere after that. They really didn't know much about lethals, but they did know they didn't want to lose future foals from this cross, so it never happened again. Since both men just liked to dabble in breeding, mostly for their own enjoyment, they raised a foal or two per year to play with, and then sell to help offset some of the costs of keeping their adult horses. Not wanting to cause any problems, neither spoke of his horse parenting a dead foal when it came time to market Beauty & Hotstuff. Some nice gal just fell in love with Beauty, and another fellow thought Hotstuff was pretty sharp, so bought him to put in his breeding barn one day. So, off the foals went, in two separate directions, with a pretty reasonable chance of not ever meeting. The gal shows her mare for awhile and after doing some winning, decides to retire her to broodmare...hoping to create some winning babies of her own. She toys with this and decides she might really like a foal, so off she goes in search of the perfect mate for Beauty. She hears of a good stallion some 100 miles away who is doing a fairly decent job on his foals....and when she sees....guess who!!...Hotstuff, she falls in love with him. The deal is done---the potential danger unbeknownst to either party. Neither knew that both Hotstuff's and Beauty's parents were lethal carriers...Mr. Jones or Mr. Miller neither one said a word about it. What did she get for a foal? Let's look at the possibilities. Since both Beauty and Hotstuff are single-gened tobianos, there is potential for a TT foal, a Tt, or a tt--homozygous, heterozygous, or non-tobiano. But, since the overo gene is separate and devoid of any connection to the tobiano, and is present in both Beauty and Hotstuff, it, too, can go to the foal. She could then end up with a solid or overo with this gene....or a dead foal. Both Beauty and Hotstuff were begot from carrier parents....and since neither horse had been tested for lethal, neither this gal or Hotstuff's owner knows if their horses carry the gene, too. Nothing would stop the lethal from going to the foal...only Mother Nature controls that. So, with the tobiano genes doing their things, and the overo/lethal doing their things...this is what could happen. If both parents passed on tobiano, the foal would be homozygous for tobi pattern--GREAT!! But if both parents also passed on the lethal that has theoretically passed on to Beauty and Hotstuff (for the purposes of my illustration, here), then this same TT foal is going to die, regardless of how much tobiano pattern it has, how many pawprints, or what have you. The overo is separate of the tobiano gene, and so is the lethal. So no matter what the tobiano does, the lethal can still slide in there and kill the baby with the classic gut problems that do the killing. Granted, there is only 25% chance of this foal being a dead one, but this same chance exists every time this cross is made. What would this kind of foal look like? Lots of tobi spots? Pawprints? Very few tobiano characteristics? Who knows....no lethal TT foal has ever been documented or studied because anyone who had one didn't know why their tobi foal died. Since these foals are gone, we now have to document our breeders to be able to avoid this. I can only hope that breeding doesn't go on purposely with these carriers for research reasons...what a waste to intentionally kill an innocent foal. So, test your horses, breed responsibly, and try to avoid the now needless deaths of who could have been your next APHA champion! We would like to thank Carolyn Schultz for this important and educational article. Carolyn also wrote the article Dispelling The Myths of TOVERO Paints.
Carolyn Schultz is also the author of "Understanding and Breeding for the
Homozygous Tobiano". This is the ONLY comprehensive work of its kind, in
everyday language, so all can understand.
Carolyn Schultz |