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Incomplete dominance and codominance definitions tend to blur together, but we’ll clear it up here with in-depth definitions and useful tips.
It’s a classic trap: both terms deal with heterozygotes, and both lead to phenotypes that look different from either parent. But how they differ and how the MCAT tests them matters.
You’ll find these in the Biology/Biochemistry (Bio/Biochem) section, usually in genetics passages or experimental setups testing allele interactions. Getting these concepts straight is crucial for interpreting results in inheritance patterns and phenotype ratios.
In other words:
Use this as your mental sorting tool any time inheritance patterns come up.
A go-to MCAT example is snapdragon flower color. Let's say the red flower color is encoded by allele R, and white by r. In a typical dominance scenario, you'd expect red to dominate white. But here, R is incompletely dominant.
Why pink? Because the red allele can’t fully mask the white, so the result is a blended color. This blending is the hallmark of incomplete dominance.
A textbook codominance example is ABO blood type inheritance. The IA and IB alleles both produce distinct proteins (A and B antigens) that appear on red blood cells. When a person inherits one of each:
Unlike in incomplete dominance, there’s no blending here. A and B antigens are both present and fully expressed. You could test for them separately and detect both with no merging—clear codominance.
Analogies help a lot when you’re trying to mentally categorize these tricky concepts.
If that doesn’t click, another analogy is cow patches:
This is a question that trips up a lot of students:
“Would patches of red and white count as incomplete or codominance? Within each patch, one allele would express completely, right?”
If the phenotype shows distinct patches of red and white (and not a pink blend), then you're seeing codominance. Each allele is being expressed in its own territory, like regions on a flower petal or fur on an animal. That’s not a mix—it’s both traits showing independently.
Some people might confuse this with mosaicism, where different cells express different genes due to mutations or X-inactivation. But unless you're diving deep into cell lineages, this isn't an MCAT concern. For test purposes, patches = codominance.
Watch out for these on the test:
You won’t get a simple “Define codominance” question on the MCAT. Instead, expect to see:
For example:
A plant with pink flowers is crossed with a red-flowered plant. What’s the expected phenotypic ratio in the offspring?
That question hinges on recognizing incomplete dominance—you have to realize that pink isn’t a dominant color but a heterozygous blend. Likewise, if a question describes a blood sample showing both A and B antigens, that’s codominance in action.
Mastering the difference between incomplete dominance and codominance is crucial to nailing the BBLS section of the MCAT. It’ll help you interpret tricky passages, get inheritance questions right, and move quickly through genetics-based reasoning.
Reviewed by:
Neurologist, Touro University Nevada College of Osteopathic Medicine
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