How can co-dominant alleles work in flower petals?

Question

I am studying gene inheritance, and I understand complete dominance/co-dominance and incomplete dominant penetrance in flowers. I know how to recognize each case.

However, how does co-dominance work? If I understand correctly: when a gene is transcribed, both alleles are transcribed. Hence, for flower petals, if I have a "red" allele and "white" allele, and there is incomplete dominant penetrance, I get rose petals. However, if both alleles are co-dominant, why are petals either red or white? Aren't both alleles transcribed? Or perhaps, in the case of co-dominance, heterochromatin is responsible for shutting down one allele in lineage cells?

I hope that I am expressing myself correctly. Thank you very much for your time!

Answer 1

We tend to use "incomplete dominance" to describes situation of haplo-insufficiency. The white flower petal has two broken alleles for making pigment, and one working copy doesn't yield enough pigment to make the flower look as red as it would if it had two working red pigment alleles.

"Co-dominance" is for two functioning alleles when the effect is that you can see them each, and the result is not an intermediate blend. Like having a working A antigen and a working B antigen. A blood will not clot when mixed with A blood. AB blood doesn't halfway clot, it also won't clot at all, just like A blood.