Recessive- a PAIR of mutant alleles at a given locus is required for a visual change to occur. An animal without a single allele of the trait will look completely normal. In the example illustrated, the purple smiley is homozygous for a recessive trait and the white smiley is a wild type normal. All the offspring would appear normal, even though they would all carry a single mutant “purple” allele.
Dominant- only ONE allele is required and the result looks exactly the same as if two were present. In the illustration, we mate a “Super Orange” to a normal. The super orange is homozygous for orange, so it will pass on an orange allele to every one of its babies. Even though the baby has only one orange allele, it looks exactly like its homozygous parent, who has two orange alleles.
Co-dominant- only ONE allele is required to produce a visual change. In the case of co-dominance, the two traits are more or less equal and the visual appearance of the offspring reflects a mixture of the homozygous forms of each allele. For example, a red flower crossed with a white flower would produce a flower with both white and red petals.
Incomplete dominant- only ONE allele is required to produce a visual change. In the case of incomplete dominance, the traits are both expressed. The visual appearance reflects a blending of the homozygous forms of each allele. For example, a red flower crossed with a white flower would produce a PINK flower…or a homozygous pastel (super pastel) crossed with a wild type (normal) produces offspring that appear in between the two (pastel). It's pretty obvious that the pastel offspring is not a normal, but it is also easily distinguished from it's homozygous (super pastel) parent.
You’ll also hear the term co-dominant applied to traits like spider and pinstripe, which is also incorrect. Neither of these traits can exist in a homozygous form. We are unsure why, but no homozygous spider or pinstripe has ever been produced, despite a decade of opportunity and dozens upon dozens of pin x pin and spider x spider matings. Without knowing for sure what a homozygous pin or spider looks like, we must regard these traits as functionally dominant.
Most ball python breeders didn’t set out to be geneticists, but as more and more morphs are discovered and mixed, acquiring a vocabulary to describe inheritance becomes necessary. Incorrect terms were used by many originators, because they honestly didn’t know any better. Times have changed. The information is freely available on the internet, taught in most high school biology classes, and it’s really past time we started using the jargon correctly.
What harm does is cause to use the word “co-dom” even though the trait is really an incomplete dominant? Well, really- none- as long as everybody understands what you mean- and that is where the trouble starts. Is it really so hard to start using the right word once you understand it? Utilizing the correct terminology will allow better communication and understanding of more difficult genetic issues.