Hello all!
This is a simple introduction to genetics, in which I'll go over a few terms. It may seem boring, but it's extremely vital to understand these terms. Let's start with what genetics really is. If you look up a definition of genetics, you'll get some fancy scientific sentence about inheritance and characteristics. Don't let that confuse; genetics is literally just the genes making up your body.
That leads do the next thing - what is a gene? A gene is a building block of the entire body. It is basically information stored in your body to determine several things; one thing in particular is the coloration of an animal. That is determined by genes, as well as size, shape, etc. The coloration is what we'll be focusing on here. Think of a gene as a blueprint. That blueprint determines how something is going to function - that's essentially all a gene is.
Now that we've discovered what a gene is, we can dive into the terms of genes. Two very important ones are genotype and phenotype. Genotype, as may be indicative of the name, is the genetic make up. A phenotype, however, is how an animal looks on the outside. I'll throw in an example, but don't fret if you don't get it! A certain cat will have a genotype of BB and a phenotype of simply black. Another cat will have a genotype of BBdd, but a phenotype of blue.
How do we determine where all these letters are coming from in the genotype? Well, we use alleles. Each letter used above (BB and BBdd) are alleles. An allele is a variation of a gene. All animals will have two alleles on each locii. Wait now, what is that? A locii is basically a single point on a strand of DNA in which two alleles join together to make a gene. So, in our above example of a black cat, it is displaying BB. B is an allele, B is an allele and they are both on the black factor locii to create a genotype of BB and a phenotype of black.
Now that we have all that scientific mumbo jumbo out of the way, let's focus on a few things important to determining a phenotype. One thing to know right off the bat is dominance. There are two ways an allele, for a very basic way of looking at it, can display. One is dominant and one is recessive. As is indicated by the names, dominant is what is displayed over recessive. To display a recessive, you need two recessive alleles to create that recessive phenotype. Now remember the blue cat we had way back when? Blue is a recessive of black. In order to get that you need a black factor on and then a dilution on. BB was our genotype for a blue cat, meaning that it's base color is black. BB is dominant black - dominant genes are usually portrayed as a capital letter. This means this particular cat has two dominant alleles and will not carry a recessive. If this particular cat was Bb, it'd be carrying a recessive; however, it'd still be black because dominant only needs one gene to make it show as the phenotype. So that brings us to the next set of letter. We know we have a black cat right now, but what does dd mean? dd is a recessive allele paired with a recessive allele in the dilution locii. Because this particular cat has both recessive alleles, its phenotype will show dilution. That makes it a blue cat. A recessive gene needs both alleles to be recessive, unlike the dominant alleles.
Now it's not always this simple. There is a thing, which I will dive into much later when we reach that point, called co-dominance. Another term you may see for this is called incomplete dominance. Basically this means that you need two different copies of alleles to create a mixture of these alleles for the genotype. A common example is roan in cows/horses. A roan in a horse will display if they have an R' gene and an R gene. R in horses is a solid color and R' is an unpigmented color. One would naturally assume that one has to be dominant and one recessive - this is not necessarily the case. When you get a blend of RR' then you get a roan. Which is neither solid nor unpigmented, but a mix. They have some unpigmented hairs and some solid (or pigmented) hairs, creating a beautiful blend of both that is a unique appearance compared to either or. This will only display when you have a mixture of the alleles, not when you have two dominants or two recessives. That is co-dominance.
The last thing I want to cover for this introduction is heterozygous and homozygous. Remember how I was speaking of having two dominant or two recessive alleles? Well, these terms tie into that. Heterozygous means having two different alleles. One recessive and one dominant. For example, if the previous cat had been Bb, that is heterozygous. Because it was BB though, that means it is homozygous. Homozygous means it has two of the same alleles, such as the example of BB. It has two dominant alleles, making them the same allele.
And that's an introduction to genetics! Hopefully this gives you a better understanding of the basics and we can start diving into the meat of it.