Recessive type genes don't produce a visual change unless the animal is carrying the full pair of the mutant gene. An example is Piebald. A "Het" Piebald Ball Python looks just like a wild or normal Ball Python. When a breeder is not sure if the animal is carrying a recessive gene, they can refer to it as being "possible het", possibly with a percentage of likelihood based on the parents (e.g., "66% possible het").
Genes which are Dominant produce visual effects even in the heterozygous form. This means offspring with only a single parent carrying the gene can have a mutated appearance. In true Dominance, the same altered visual effect is achieved with either a single gene or pair of genes. The Pinstripe in Ball Pythons is a well known Dominant gene, which is indistinguishable in het or "super" (homozygous) form.
Incomplete Dominance is when the heterozygous and homozygous forms each produce different morphed appearances. For example, the Lesser Platinum gene produces a lighter colored appearance in homozygous form, and the solid white Blue Eyed Lucy (BEL) in homozygous form. Breeders who discover a new Dominant gene cannot say that it is Incompletely Dominant until they produce a super form which differs from the Heterozygous one. Without knowing if this is even possible, it can take many generations to "prove out" the gene's true nature.
The Reptile community has commonly used the term Codominant to refer to genes that produce a different appearance in homozygous and heterozygous forms. Incomplete Dominance is however, a more accurate biological term in most cases. Incomplete Dominance describes genes that produce a blended effect. For example, red and white genes producing pink (or any other color) are incompletely dominant. By contrast, Codominant describes genes which produce an unblended effect. An example would be red and white genes producing an appearance of red and white. It might appear to be pink at a distance, but it is really comprised of white and pink hairs, scales, etc. Most of the reptile genes which have been referred to as Codominant have the blended effect, and thus would be more accurately categorized as Incomplete Dominants.
Here's a short video which illustrates these differences.
Where do forms of Dominance show up on MorphMarket? For each species, you can browse the ads by traits or morphs. On this index page, you'll find the traits color-coded by dominance. Warmer colors represent recessives and cooler colors represent the dominants. We further divide the ads using different colors for possible, het and homozygous or super forms. Grayscale indicates traits that are not directly inheritable.
You can find learn more about genetics using the links on our resources page.
(Flower photo by ForestWander.com used under Creative Commons 3.0)