Posted by:
Paul Hollander
at Tue May 30 16:48:55 2006 [ Email Message ] [ Show All Posts by Paul Hollander ]
>Codominant and incomplete dominance are totally different.
Well, they are similar in at least one way. In both, the heterozygous form can be distinguished from the two homozygous forms. And that is why we can use "incomplete dominance" and "codominance" as synonyms. I prefer "codominance" because it takes less typing.
>Incomplete dominance shows a mixing of phenotypes of the two genes in the heterozygous form, hence the pink flower from a red and a white.
>
>Codominant shows BOTH phenotypes simultaneously. If the red and white flowers are codominant, you will get red and white petals in the flower as opposed to a mix. If the petals form the same, then you will have white and red flowers on the same plant. Either way, you get both exact phenotypes and not a mix.
Part of the difficulty is that "codominant" came out of the discovery of A, B, and AB blood types, molecular level testing. "Incomplete dominant" and several other terms came animal and plant breeding, where phenotype was on the organism level as determined by the unaided Mark I eyeball. Molecular level testing is far more sensitive than the eyeball. That's why they can pick up both the A and B phenotypes in AB individuals.
When you get down to the molecular level, it turns out that heterozygous cells produce a mixture of two gene products. One gene product is the same as that made in one parental homozygous form, and the other gene product is the same as that made in the other parental homozygous form. Note that this is within each cell, not one product in one cell and the other product in a totally different cell. The intracellular mixture of products is why you can get an intermediate phenotype on the organism level in both codominance and incomplete dominance.
I'm not denying that there are some differences between incomplete dominance. But from what I've found, it is primarily in functionality and the quantitative effects of the two gene products.
As the molecular genetics boys get their innings, I think there will be some changes in the way some of the standard terms get defined in the texts.
>This is why dominant boa morphs would be classified as incomplete dominance and not codominance. (snip)
This does not compute. Standard operating procedure in genetics is to call a dominant mutant gene a dominant mutant gene. A dominant is neither an incomplete dominant nor a codominant.
Paul Hollander
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RE: co-dom vs. dominant - Paul Hollander, Tue May 30 16:48:55 2006
