This page will cover the basic genetics terms and concepts you will need to understand the rest of my articles. It doesn't go into how to calculate offspring - I may put that on a separate page later, but I am planning to make a video to explain it.
A gene is one section of DNA. For example, we have a gene known as Lac, which makes a person either lactose tolerant or lactose intolerant. If there are multiple "versions" of a gene, they are known as alleles. The Lac gene has two alleles, one for lactose tolerance, and one for lactose intolerance. I am lactose intolerant, but I still have the Lac gene, I just have the specific version of that gene that makes me lactose intolerant.
A locus is the place in the genome where a gene is located. The plural is loci. I will not use this term much, since it is often synonymous with gene.
Animals and some plants have two copies of every gene, with one copy inherited from each parent. Genotype is the combination of alleles that an individual has. This is contrasted with phenotype, what the individual actually looks like.
To see how these are different, imagine a gene that makes a flower red or white. This flower can either have two red alleles, two white alleles, or one of each.
When it has two red alleles, it's red, and when it has two white alleles, it's white. In either of these cases, it is homozygous, meaning it has two of the same allele. But what if it is heterozygous, having one of each allele? There are a few different ways it can go.
It could be completely red. If this is the case, we say that the red allele is dominant to the white allele, and the white allele is recessive to the red allele.
Or it could go the other way - if our heterozygous flower is white, then white is dominant and red is recessive.
If the flower looks intermediate between the two colors (pink), we have a case of incomplete dominance. Since it is an intermediate, it doesn't matter which allele we say is dominant or recessive - either way, the heterozygote is pink.
Or, the flower could have patches of both red and white. In this case, where both options are present, the two alleles are codominant.
Not every trait is particular to one gene, of course. Traits that are influenced by many different genes are known as polygenic traits. They usually have a spectrum of possibilities. An example is skin color in humans. With polygenic traits, the most important thing to know is that offspring will most likely fall in between their parents, but not always.