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Basic
Genetics
Horses, and all living things, contain genes which act as blueprints to
build the animal. Everything about you or your horse-- your height, hair
color, eye color, nose size-- everything is written in your DNA.
All the genetic material is contained in Chromosomes. Basically, DNA combines with proteins to form tightly-wound genetic molecules. These Chromosomes (there are 32 pairs in the horse) are made up of smaller parts called genes. Each gene is found in the same spot on the same chromosome in every horse. Now, these genes are not just randomly placed on the chromosome; every gene has it's own spot on the chromosome. This particular spot that the gene is found on is called its locus. The plural form of locus is loci.  There are different variations of genes. These are called alleles. For example, in the horse there is a gray gene. There are two forms of this gene: either G or g. One allele, G is the 'gray:on' gene, which makes the horse gray. It codes for a gray horse. Uncapitalized, g stands for the 'gray:off' allele of the gene, meaning the horse is not gray. It codes for a non-gray horse.  Capital or uncapital letters stand for different genes. Capital letters stand for the dominant, or 'on' gene; uncapital stands for the recessive, or 'off' gene. You'll find a list of every gene's symbol and functions under the A to Z gene section. Usually, when you're writing about a horse, you will only write the dominant genes, because they are the ones that show up in the horse's color. For example, people say a chestnut is ee, but that doesn't mean he has only these two genes. A chestnut horse also has gg, which are two gray 'off' genes, and every other 'off' gene. But that is a given, and doesn't need to be written; usually only the color-affecting genes are written, unless there is some question as to whether or not a horse possesses a gene (for example, if the horse was a roan but looked gray, one might write "gg" when describing the horse's genetic makeup, to clear up any confusion). A series is a list of all the types of alleles for a particular characteristic. In horses, there are usually two forms of a gene (alleles), what I call the 'on' and 'off' forms. One codes for a particular color, and the other does not. For example, the Rn gene codes for a horse to be roan; the rn does not. All non-roan horses are rnrn. More terms you should know: phenotype is what a horse looks like; and genotype is his genetic makeup. Two horses with identical phenotypes, or physical colors, can have different genotypes, or genes. Genes are referred to as either dominant or recessive. Some genes are more dominant than others-- they override the other genes and have the final say in what color a horse will be. Recessive genes will be overridden by dominant genes, if the dominant genes exist in the horse. The most recessive color is chestnut (also called sorrel)-- it is recessive to every other color. This means that a red horse will have no dominant genes 'on' only his chestnut genes... otherwise he wouldn't be chestnut, he would be overridden. Chestnut bred to chestnut always gives chestnut, since those are the only 'on' genes they have to pass on. The most dominant color is gray. Gray overrides every other color-- a horse could have every other gene, and just one gray gene would override the entire horse. He'd be a gray. Every gray horse has a gray parent-- you can't get gray from non-grays. Why? Because it's the most dominant-- horses cannot have 'hidden' gray genes, because if they did have a gray gene, they would be gray, as gray is dominant over every other color. Now, every horse has TWO alleles of a gene. Why? Because chromosomes come in pairs, one from each parent. And on each chromosome is a locus for a specific gene... such as the gray gene. That means there are two total loci for each gene in the horse. Since every horse has two loci for any gene, that means that they can have both the same alleles at each loci, or different. What does that mean? Well, a gray horse could be Gg or GG-- and just by looking at him you wouldn't know which. He could have one, or two, dominant gray genes. A non-gray horse is gg-- he doesn't have a gray gene, so he's not a gray. A horse that has two alleles of the same gene is called homozygous. Both his genes are the same, so that means he will be sure to pass them on-- he can't not, since he has no other genes. A GG gray horse's offspring will ALL be gray. A horse with one of each is called heterozygous. While a heterozygous gray horse will be gray, this doesn't mean all his foals will be-- only 50%, since he has an equal chance of passing either his G gene or his g gene. Here is an example of the genes for fictional horses named Bob, Betty, and Fred. You see, each horse has two dun genes; they can both be on, they can both be off, or they can be one of each.  The first horse, Bob, is dd, which means he has two non-dun coding genes. In other words, he's not a dun. If he is bred to non-dun mares, he will never produce a dun. The next horse, Betty, has one dun gene and one non-dun gene-- and since the D gene is dominant, she's a dun, too. But wait! What about that non-dun gene? Doesn't it code for not dun? Well, the 'on' form of the gene is dominant, so she's still a dun because her D gene says so. Her foals will either get the d gene from her, or the D gene from her. So, she has a 50% chance of giving her foals dun coloring. The last horse, Fred-- he's a homozygous dun. He has two dun genes. While he may look identical to Betty, in other words his phenotype is the same, his genotype is different. No matter what mares you breed him to, he'll give the foals a dun gene-- because that's all he has to pass on! When a gene at one locus has an effect on a gene at another locus, we say that it is epistatic. The gene that is affected is hypostatic. For example, the G (gray) gene is epistatic to the ee (chestnut), and all other genes, because it hides them. Interitance: Horses inherit 50% of their genes from their mom, and 50% from their dad. Both the sire and dam gives one allele of each color. They contribute equally to the genetic makeup of the foal. Now, which gene does he or she give (they have two genes, but only pass on one)? Well, there's a 50% chance of either one. When you flip a coin, it can land either heads or tails up. There's a 50% chance of either happening. If you flip that coin 10 times and you get heads each time, does that mean the next one will be heads, because there's a trend for that coin, or does it mean the next one will be tails (it's about time!)? No, neither conclusion is very reasonable. When you flip a coin, regardless of past flips, you still have an equal chance of each. The coin doesn't know which way it landed and make a decision to land another way; it's chance. Many people foolishly think that past flips somehow affect the future ones-- not just in coins, but in horses. If your mare has a filly, does that mean she will have a colt next time? If she has 5 colts in a row, does this mean she's a 'colt-producer', and will always have colts in the future? NO! Every time she's bred there's an equal chance of either sex being produced. Oh, and by the way, the sex of the foal is actually determined by the sperm... so it's not the mare's fault at all, NOR is it the stallion's. It's just like flipping a coin-- it's chance. Because there is a 50% chance of either one, does this mean that if you flip a coin 100 times, 50 will be heads and 50 tails? No, it doesn't. You cannot accurately predict which specific way the coin will land, but you can tell that there is an equal chance of each. Horse genetics is much the same way; there are many times when you can tell for sure what color the foal will be, but there are other times when you can only give predictions and say which colors he is most likely to be, and which colors he cannot be. There are always unknowns which will jump up and surprise you, though rarely-- and with an understanding of horse genetics, you will be able to tell, in most cases, with 99% accuracy what color your foal will be.
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