DNA, or deoxyribonucleic acid, is the genetic material of almost all living organisms. This material is essentially a long string of information which provides the instructions for how to make the organism. There are four bases of DNA, adenine, cytosine, guanine and thymine, commonly called just ‘A’, ‘C’, ‘G’ and ‘T’. These bases can be arranged in code to form the genome.
Fun fact: The full set of instructions is called the genome. The horse’s genome is about 2.7 billion base pairs!
Genes are simply segments of the genome which encode for some product (either RNA or protein). Diploid animals, like horses, have two copies of every gene.
Fun fact: The genome is only partially composed of genes. The rest is “non-coding” DNA. Previously, this non-coding DNA was called “junk DNA” by scientists, because it was thought to have no purpose, but this has since been proven wrong. I won’t go into details on that because it has no role in this game, just know that it does stuff!
Horses all have the same genes, so what makes each horse different? Well, the code within each horse’s two copies of a gene can vary. These variations are called alleles.
Think of it like this: You asked two different people to make a turkey sandwich. One made a sandwich with turkey, tomato, lettuce and mayo. The other used turkey, cranberry chutney and spinach. Both are turkey sandwiches, but different. Think of the turkey sandwich as being the gene, then the variations are the alleles.
Fun fact: A gene is not limited to having two alleles at the population level (meaning if you looked at every single horse in the world, a gene may have more than two alleles). Some genes even have dozens of alleles!
A genotype is simply a representation of the specific alleles a horse has for each gene. These are commonly written as a single letter or a single letter with superscripts. For example, the extension gene has two alleles, commonly written as E and e. A horse can then have a genotype of EE, Ee, or ee.
A phenotype is an observable characteristic. Examples include: blue eyes, bay coat color, sickle hocks, and so on.
This is referring to a genotype. Homozygous means the horse’s two alleles are exactly the same. I.e., in the case of extension, the genotype of either EE or ee, and not Ee.
This is referring to a genotype. Heterozygous means the horse’s two alleles are different. I.e., in the case of extension, the genotype of Ee, and not EE or ee.
When an allele is dominant to another allele, it will “mask” the effect of a recessive allele. Meaning the recessive allele will only be partially noticeable or not at all. There are two types of dominance though: complete and incomplete(aka partial). When an allele is completely dominant to another, then the other allele’s effect is completely masked. If an allele is incompletely dominant, then the other allele’s effect will be partially visible (think bay-buckskin-perlino).
Important note: If an allele is dominant to one other allele, that does not mean it is always dominant in every genotype. We mentioned some genes have more than 2 alleles. Sometimes allele 1 can be dominant to allele 2, but recessive in comparison to allele 3, and so on.
Diploid means that organism has two copies of each chromosome. This is where the two copies of each gene comes from. Horses are diploid, as are most animals.
Gametes are the germ cells, either oocytes (eggs) or sperm. Unlike the parent, each gamete is haploid (contains one set of chromosomes) rather than diploid. Once a single sperm and oocyte fuse to create an embryo, the sets of chromosomes will combine to make a diploid embryo.
Get that image of crime-fighting turtles out of your head. In reality, we’re all mutants. Dun dun dunnnn.
Mutations are simply changes or differences in one’s genome. That’s it. They can be a single base, or many bases long. We all have them, otherwise there would be no alleles, and we’d all look exactly the same.
Where do mutations come from? They can be generated by many things, ranging from a “mistake” made when your cells divide, to damage from things like UV rays. Every cell in an organism can have the same mutation, or sometimes it can be limited to a few cells or just one cell. If the gametes happen to have the mutation, it will be passed on to future generations. If not, then the mutation will not be inherited and only affects the individual.
Mutations can be several different things. They can be small, such as a simple base swap (a.k.a. A single nucleotide polymorphism (SNP)), a deletion or insertion of one or more bases, or an inversion of two or more bases. If you are a visual learner, here is an example of each.
Mutations are not always bad. But, they’re not always good either. Sometimes they are neutral too (i.e. they have no visible effect). Life is complicated.