Miscellaneous Notes
by C. Williams

Whew! You've made it this far! And there's still more? You bet... but for this discussion, there's just a couple more tidbits to share, to help fill in a hole or two.

Polygenic Traits are those things that are determined by several loci working together... in which the exact loci and genetic mechanisms aren't known. Poly= many and Genic= of the genes. The Epistasis between the Agouti and Extension loci are an example of a polygenic trait-- however, some traits likely involve more than just two loci-- they may involve 3 or 5 or more, and perhaps with several possible alleles apiece.

Examples of Polygenic traits in reference to Horse Coat Colors would include blue eyes, and white markings.

Of course, the final coat color you see on a horse is a Polygenic Trait all by itself. More than 20 known or hypothesized loci have been documented so far to determine the final color you see. Some of the alleles have such obvious effects (such as grey and cream) that it is very easy to identify what alleles the offspring are carrying and pinpoint the gene action of small parts of the color puzzle. We still don't know what loci, alleles or inheritance patterns determine whether a horse will be a light, medium or dark shade of a color, or what causes some horses to be more metallic and others less. But with continued research, we may be able to discover more details in the future.

All physical traits are determined by some type of genetic mechanism. Traits which we know are heritable but we don't know the exact mode of inheritance for are usually assumed to be polygenic in nature, as there really aren't many ways the alleles at a single locus interact. But when you combine a half a dozen different alleles... that's another matter in sorting it out. For now, geneticists must rely on phenotypic evidence. But with gene mapping projects underway for many species, some day, researchers may be able to more quickly identify loci and alleles that will help explain inheritance patterns of many more traits.

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Mutations are nothing more than a "blip" in the DNA. During meiosis (and also mitosis) where a cell replicates its own genetic material, every now and then there is a "typo" so to speak. That typo may have no effect, or may have a very noticeable effect on the animal that receives it.

The rate of mutations, or how often they are likely to occur, is known to be accelerated when the body is subject to some things... certain chemicals and radiation are two of the most notable mutagenic factors--things known to increase mutation rates dramatically.

Mutations tend to be random. Researchers believe that some chromosomes or some areas of chromosomes tend to have higher a higher occurence of mutation than others. One wrong amino acid in a chain can cause a new coat color to appear. It can also cause the animal to be born with a lethal defect, or to have some type of change occur that cannot be easily seen or noticed... and it may have no effect at all. It stands to reason that mutations that aren't a liability to the animal, are likely to be passed on to future generations.

According to Darwinian theory, evolution is based on mutations that are advantageous to the animal, and help it to survive better than it's peers. An animal that thrives, usually lives long enough to reproduce itself. Mutations that cause a problem often are lethal, and are never passed on. Most researchers believe that far more mutations are harmful, and those that are helpful to the animal, or may just be innocuous, are far less likely.

Mutations occur even now, although most of them may never be noticed. It is believed that Icelandic horses, whose populations have been isolated from outside influences of new genes for centuries, may have a special type of pinto patterning... the result of a mutation that occurred after those horses were brought to Iceland. Not much is currently known or documented at this time, but likely more information will come about in the next few years, since horses of color and interest in coat color genetics has increased of late.

Perhaps the most noted modern-day mutation, is in regard to the Quarter Horse stallion "Impressive". Studies on this horse's offspring have lead researchers to believe that this horse is the original bearer of a particular mutation that causes a condition known as Hyperkalemic Periodic Paralysis or HYPP The condition is caused by the presence of a mutant dominant allele and unfortunately, the result is not just a pretty new color variation.

It is my opinion, that likely many of the interesting colors we now see on the domestic horse, are likely to be the result of mutations somewhere along the way. The original "wild type" horse is believed to closely resemble the Przewalski Horse (of Asia)-- whose color is fairly close to "wild-type bay" in the domestic horse. Domestication most likely allowed "unusual" colored animals (the result of a mutation at a particular allele) to be noticed by humans. Their unique color may have caused their keepers to consider them "special" in some way, and it's likely they were allowed or encouraged to propagate themselves, and pass the mutation to future generations.

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Views expressed herein are those of the writers and compilers of the various information. Reference sources are cited where applicable. Copyrights are the property of the respective authors.