Overdominance - Wikipedia
Overdominance, or a fitness advantage of a heterozygote over both homo- zygotes, can of the relationship between phenotype and fitness, and the constraint caused by static nature of fitness under stabilizing selection, even for traits. In Mendel's work on pea plants, each gene came in just two different versions, or alleles, and these alleles had a nice, clear-cut dominance relationship (with the. Indeed, in a few examples, a trait that shows overdominance sometimes of each allele for a specific phenotype under certain environmental conditions.
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.
What are Dominant and Recessive?
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.
- Co-dominance and Incomplete Dominance
- Multiple alleles, incomplete dominance, and codominance
- "Explanations" of Male 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.
Co-dominance and Incomplete Dominance (video) | Khan Academy
Now what incomplete dominance is, is when the heterozygous phenotype shows a mixture of the two alleles. Some rats have a single gene mutation that makes them resistant to Warfarin that is inherited as autosomal trait. These rats tend to die of Vitamin K deficiency. Heterozygotes for the Warfarin resistance allele are still resistant to Warfarin but only need 2 or 3 times the normal dietary Vitamin K to stay healthy.
These heterozygotes are overdominant for survival. At this locus there is a resistance allele A1 and a wildtype non-resistance allele A2 A2A2 homozygotes are killed by Warfarin.
A1A1 homozygotes are resistant by Warfarin but die due to Vitamin K deficiency. Codominance is perhaps the hardest dominance concept to understand.
What are Dominant and Recessive?
With codominance, the heterozygote has the phenotype of both of the homozygous parental lines simultaneously. At the most basic biochemical level, all structural genes are codominant. Each gene codes for a protein. Different alleles of that gene code for slightly different proteins.Barney Stinson - Life Lessons (How I Met Your Mother)
A heterozygous individual would make both types of proteins. That is, the heterozygote has both parental phenotypes. Roan color is cattle is often described as a codominant traits. It is possible to get homozygous red cattle and it is possible to get homozygous white cattle.
When these cattle are crossed to make a heterozygote, you get a cow that has both red and white hair patches.