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Co-dominance and Incomplete Dominance

In genetics, there are three main dominance patterns: complete dominance (only one dominant allele appears in the phenotype), co-dominance (both alleles are visible in the phenotype), and incomplete dominance (a mix of alleles creates a new phenotype). These patterns help us understand how traits are passed down from one generation to the next. Created by Ross Firestone.

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Video transcript

- [Voiceover] So today we're gonna talk about Co-Dominance and Incomplete Dominance, but first let's review the example of a blood type and how someone with the same two alleles coding for the same trait would be called homozygous and someone with different alleles would be called heterozygous. Also remember, the concept of dominant and recessive alleles and how the A allele is dominant over the O allele in this example. This means that the same phenotype, blood type A, can result from these two different genotypes. Now, the example that I just gave you was an example of Complete Dominance. So if a person had a genotype AO, since our phenotype is just blood type A, it means that the A allele is completely dominant over the O allele and only the A allele from the genotype is expressed in the phenotype. But there are actually three different patterns of dominance that I want you to be familiar with and to explain this I'm going to use a different example. Let's say we have this flower and the red petal phenotype is coded for by the red R allele and the blue flower phenotype is coded for by the blue R allele. So I'm going to introduce three different patterns of dominance and they are complete dominance, which you've already heard of, co-dominance, and also incomplete dominance. I'm going to explain what these two new patterns are through this flower example. Let's start by looking at three different genotypes and the phenotypes that you would see for each of them under each different dominance pattern. We'll start with the genotype, two red Rs, which we could expect that in all cases the flower petals will be red since we only have red Rs in the genotype. Similarly, if our genotype had two blue Rs then we could expect that in all cases the flower petals will be blue since we only have blue Rs in the genotype. Now these three different dominance patterns change when we look at the heterozygous example. That's what makes these three patterns different. Now we're already familiar with the example of complete dominance, so if we said that the red R is dominant over the blue R then this would make the heterozygous phenotype a red flower for complete dominance. Now what co-dominance is, is when the heterozygous phenotype shows a flower with some red petals and some blue petals. So it's when the two alleles are dominant together they are co-dominant and traits of both alleles show up in the phenotype. Now what incomplete dominance is, is when the heterozygous phenotype shows a mixture of the two alleles. So in this case the red and blue flower petals may combine to form a purple flower. Neither allele is completely dominant over the other and instead the two, being incompletely dominant, mix together. So what did we learn? Well, if we assume the heterozygous genotype, red R, blue R, then there are three different dominance patterns that we might see for a specific trait. In complete dominance, only one allele in the genotype, the dominant allele, is seen in the phenotype. And this was the example with the red flower. In co-dominance, both alleles in the genotype are seen in the phenotype. This was the example with the flower with both red and blue petals. Finally, in incomplete dominance, a mixture of the alleles in the genotype is seen in the phenotype and this was the example with the purple flower.