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Codominance - Incomplete Dominance

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

In both codominance and incomplete dominance, both alleles for a trait are dominant. In codominance a heterozygous individual expresses both simultaneously without any blending. An example of codominance is the roan cow which has both red hairs and white hairs. In incomplete dominance a heterozygous individual blends the two traits. An example of incomplete dominance is the pink snapdragon, which receives a red allele and white allele.

While most students get the idea between a dominance and recessive alleles what often throws them for a curve is the difference between codominance and Incomplete Dominance, so let's take a closer look at those, so these are exceptions to the whole idea about complete dominance where one allele will completely overwhelm or not allow the other alleles effects to be shown.

In this case rather than the heterozygotes individuals looking like the homozygotes dominance, here the heterozygote, a hybrid between two types of beings, does not look anything like the true bred ones the homozygotes so what's the difference between codominance and incomplete dominance? It's how they do this effect.

With codominance you'll see both alleles showing their effects but not blending whereas with incomplete dominance you see both alleles effects but they've been blended. Now their distinction is sometimes is hard to figure out so let me give a couple of concrete examples, so the standard example of codominance is what's known as a Roan cow. There are kind of cows that are white, there are kind of cows that are red. Now a red cow has big R big R for the hair color allele, the white cow has big W big W for the hair color allele now you may be thinking hey! I only use the capitals for the dominant why are why I'm I using two different capitals and two different letters you're supposed to use all the same letter and that's because both of these are dominant alleles so what will happen is if you have an offspring between in red and a white cow you'll get a colored cow called Roan. What happens is that you'll see white hairs and red hairs so you're seeing the effects of the white cow's hair allele and the red cow's hair allele. But you're not seeing pink hairs that would be blending and that's what incomplete dominance looks like.

The standard example of incomplete dominance is a kind of flower called a snapdragon. With snapdragons you can have red, white or pink flowers and it turns out the pink ones are blends between the red and the white alleles so if you have big R big R you'll be a red flower. If you're big W big W you'll be a white flower, if you're big R and W you'll be pink not little bits of red and little bits of white if you're getting closer and look at the flower close it's pink it's pink it's pink they blend it, so that's the big difference between codominance, they show both effects no blending incomplete dominance it's all blended.