A dominant gene can express itself when present in a single dose no matter what the other gene at the present locus is. In our example on the previous page, the full color "C" gene is a dominant gene. When genetics material is written on paper, the dominant gene is always represented with a capital letter (like the full color "C" in our example).
A recessive gene is one that must be present in a pair (that is, on both sides of the locus) to be expressed in the offspring. In our example on the previous page, the albino "c" gene is a recessive gene. On paper the recessive gene is always written using a lowercase letter (like the albino "c" in our example).
When there is a dominant gene on one side of the locus and a recessive gene on the other side, the dominant gene is the trait that is expressed. Hence, the term "dominant". In our example on the previous page, the offspring "Cc" which have one full color gene "C" and one albino gene "c" were, in fact, full color guppies because "C" is dominant and "c" is recessive.
It must be remembered that even though a fish which carries the genes "Cc" physically looks full color, the "c" albino gene is still there and will be passed on to offspring. Don't judge the book by it's cover! You have to know what the book says on the inside before you know how it will turn out!
There is a third type of gene that we need to talk about that hasn't been mentioned yet. It is called an incomplete dominant gene. In this case, the dominant gene is not able to fully suppress the effects of the recessive gene. As a result, a fish may express some traits of both genes or as an apparent blending of the two.
In reality, genes will never truly blend, so if this appears to be the case, an incomplete dominant gene may be the cause. Another reason for apparent blending is that often a completely different set of genes may be affecting the physical trait you are looking at.
You have probably noticed that we have used the same letter "C" and "c" when describing the gene pair at the same locus. The letter is arbitrary, we could just as well have used "W" and "w" or any other letter for that matter. It is customary, however, to use the same letter for both dominant and mutational gene. By doing so it is easy to keep track of which gene is the alternative (called an allelomorph in genetic terms) of the other.
Two other common genetic terms to be aware of are homozygous and heterozygous:
A feature is called homozygous when both genes express the same trait. Examples from the previous page are "CC" and "cc". Both "CC" and "cc" are purebreeding. The gene pair "CC" is homozygous normal and the gene pair "cc" is homozygous recessive.
When the pair of genes do not express the same trait, like "Cc" from our examples, the feature they represent is called heterozygous, or non-purebreeding.