BASIC GENETICS TERMS
If you already know a little about genetics then you'll probably be familiar with the following terms. Most of them crop up in the basic genetics taught at school, but just to make sure, we'll go through all the relevant ones anyway and have a look at how they are applicable to dogs.
GENES
Genes control just about everything about a living creature. They control how the creature grows and develops, how it functions and how it looks too. On this site we're focusing on just one aspect of looks - colour.
A gene is basically a set of instructions which tells the cell how to produce something (in our case, pigment). Genes come in pairs, one copy being obtained from each parent during fertilisation. A pair of genes constitutes a genotype. When we write about genes we use letters to denote them, for example a and b.
LOCII
Genes are all tied together into a long strand of DNA. Each point on this strand of DNA is called a locus (locii is the plural). At each locus there are two genes.
The genes present at each locus are picked from a list of possible genes, which is called a series. There may be any number of genes in a series, but it's usually 2-5. Each locus has its own series, and the genes in that series can only occur at that locus. For example, in the E series in dogs there are three genes - E, Em and e. Each dog has a combination of two of those genes from the E series on its E locus. It might have one copy of e and one of E, or maybe two copies of Em. See the "Summary of series" page for a list of all the series.
On this site, I will be using the terms "series" and "locus" pretty much interchangeably.
DOMINANT AND RECESSIVE
Although every dog carries two genes at each locus, only one of those genes can be expressed (meaning only one can actually be used, and the other will just lie dormant). Which of the genes is expressed depends on dominance. The most dominant gene will always be the one which is expressed. A recessive gene is one which is less dominant.
Dominant genes are generally written with a capital letter, for example B. Recessive genes are written with a lower case letter, for example b. Sometimes there is more than one dominant or recessive gene on a locus. When this happens, there is usually an order of dominance, so one of the dominant genes is more dominant than the other, and one of the recessive genes is more recessive than the other. We generally talk of series having "top dominants" and "bottom recessives".
A dog with the BB genotype has two dominant genes, and so it can only express B. A Bb dog will also express B because B is dominant over b. bb is the only genotype where the dog will express b.
It's also worth noting that sometimes both of the genes on a locus will be expressed. This is called incomplete dominance, and only happens on particular locii. The resulting dog will have a mixture of the two genes, but its markings will generally lean more towards the more dominant gene. For example, the S series controls white markings, and the genes in it display incomplete dominance over one another. si is the gene for irish spotting (white legs, muzzle, tail tip and collar), and sw is the gene for extreme white piebald (a piebald with very few patches). A sisi dog will be irish spotted, but even though si is slightly more dominant than sw, a sisw dog won't have the normal irish spotting pattern (as you would expect if a dog can only express its more dominant gene). Instead it will have a "flashy" irish spotting pattern (i.e. more white than normal), because it is able to partly express it's more recessive gene (extreme white piebald).
HETEROZYGOUS AND HOMOZYGOUS
A heterozygous pair of genes is one where the two genes are different. A homozygous pair of genes is one where the two genes are the same. We generally talk of a dog being, for example, "homozygous for merle" (two copies of the merle gene, M) or "heterozygous for merle" (one copy of the merle gene).
bB, Dd, Ayat, and swsi are all examples of heterozygous pairs of genes.
bb, DD, AyAy, and swsw are all examples of homozygous pairs of genes.