Mendel's original pea plant experiments happened to involve traits with a genuinely clean dominant-recessive pattern, one gene version fully masking the other, but that clean pattern turns out to be one option among several, not the universal rule. Incomplete dominance produces a trait that's a genuine blend of both inherited gene versions, an intermediate trait neither parent version fully expresses on its own, a red flower crossed with a white one producing pink offspring, for instance. Codominance is different again: both inherited gene versions are expressed fully and simultaneously, side by side, rather than blending or one overriding the other.
Incomplete dominance genuinely averages the two inherited traits together
When incomplete dominance is at work, neither inherited gene version is strong enough to fully suppress the other, and the resulting physical trait sits somewhere between the two, often looking like a genuine blend or average of what each version would produce on its own. A red-flowered parent and a white-flowered parent producing pink-flowered offspring isn't red "winning" partially; it's the visible trait genuinely reflecting a mix of both inherited instructions working together rather than one simply dominating.
Codominance keeps both traits fully, visibly intact rather than blending them
Codominance produces a genuinely different outcome from incomplete dominance, even though both depart from simple, clean dominance: rather than blending into an intermediate trait, both inherited gene versions are expressed at full strength, visible side by side rather than merged, roan cattle coat colour showing both red and white hairs distinctly rather than a blended pink, for instance. The human ABO blood type system is a well-known real example, where the A and B versions of the relevant gene are codominant with each other, producing type AB blood that expresses both A and B markers fully rather than blending them into some intermediate type.
What we're still unsure about
That incomplete dominance and codominance are genuinely distinct inheritance patterns, each producing predictably different outcomes from simple dominant-recessive inheritance, is well established, extensively confirmed genetics dating back over a century of careful breeding studies. What's more genuinely a matter of ongoing complexity is exactly how many real traits actually involve these patterns layered on top of still further complications, multiple interacting genes, environmental influence on how a gene is actually expressed, since a great many traits in real organisms don't follow any single clean pattern in isolation, and geneticists continue to work out the fuller, more tangled inheritance picture for specific traits rather than every trait being cleanly classifiable into one of these named patterns alone.
This sits inside Non-Mendelian Inheritance (Codominance, Incomplete Dominance), one of eight topics in Genetics, one of six domains in Biology, one of seventeen subjects the app can quiz you on.