Labrador Retrievers come in three distinct colours: black, yellow and chocolate. The colour of a dog depends on the presence of pigments in skin and hair. The pigments — which are made by specialised cells — occur as two distinct types. The first type is essentially yellow. The second one is dark, and varies from black to brown. The type of pigment that a Labrador produces depends on its genetic make-up and is the result of an interaction between two different genes, which we call the E and the B gene.
As we have already seen, a dog’s genome contains two sets of genes, one from the father and one from the mother. So for each gene a dog can have two forms of that gene — if different forms of the gene exist.
Different forms of the E and the B genes exist. In genetics we commonly distinguish between two different forms of the same gene by using uppercase and lowercase letters. So the E gene has two forms, E and e. Similarly the B gene has B and b forms.
An individual dog might receive, for instance, the B form of the B gene from one parent, and the b form from the other. With regards to the E gene it might, for instance, receive the same form of that gene from both its parents, let’s say it receives two copies of the e form. In this case, we would write a representation of the dog’s genome like this: eeBb. Quite a few different combinations are possible: EEBB, EEBb, EeBb, eeBb, Eebb, and eebb.
The form that is written in an uppercase letter is called the dominant form. This means that, if a dog has both forms of a particular gene, it is the form that is written in capitals that has an effect on the dog — it is expressed. If there is no dominant gene the recessive (lower case) one will be expressed. In the case of our example dog eeBb, the e and the B forms will be expressed.
Just how the different forms of the E and B genes interact to give the final colour is shown on the following page.