predate domestication. The mutations causing chestnut, sabino 1, and tobiano pinto are all at least 5000 years old, occurring about the same time as horse domestication. Silver and cream dilutions appeared at least 2,600 years ago, and pearl appeared at least 1400 years ago. The gray mutation is also post-domestication but thought to be thousands of years old as well. Fundamental concepts Terminology Heritable characteristics are transmitted, encoded, and used through a substance called DNA, which is stored in almost every cell in an organism. Proteins are molecules that do a variety of different things in organisms. The DNA instructions for how to make a protein are called a gene. A change to the sequence of DNA is called a mutation. Mutations are not inherently bad; genetic diversity itself ultimately comes from mutations. Mutations that happen within a gene create alternate forms of that gene, which are called alleles. Alleles of a gene are simply slightly different versions of the instructions on how to make that gene's protein. The term "allele" is sometimes replaced with the word "modifier", because different alleles tend to modify the horse's appearance in some way. DNA is organized into storage structures called chromosomes. A chromosome is simply a very long piece of DNA, and a gene is a much shorter piece of it. With some rare exceptions, a gene is always found at the same place within a chromosome, which is called its locus. For the most part, chromosomes come in pairs, one chromosome from each parent. When both chromosomes have the same allele for a certain gene, that individual is said to be homozygous for that gene. When the two alleles are different, it is heterozygous. A horse homozygous for a certain allele will always pass it on to its offspring, while a horse that is heterozygous carries two different alleles and can pass on either one. A trait that is only expressed when the gene is homozygous for its allele is called recessive, and a trait that has the same effect no matter whether there is one copy or two is called dominant. Notation Often, the dominant allele is represented by an uppercase letter and the recessive allele by a lowercase letter. For instance, in silver dapple, this is Z for the dominant silver trait and z for the recessive non-silver trait. However, sometimes the alleles are distinguished by which is the "normal"