How does the cell known which allele to express?

Question

I know you can get co-dominance, incomplete dominance... and so on.

But just take a simple dominant and reccesive allele; how does the cell know to express the dominant allele? I understand the genetics just not the physical goings on in a cell.

Help is greatly appreciated, cheers.

Answer 1

It doesn't: it expresses both but you only see the effect of the dominant allele. Suppose the dominant allele codes for an enzyme that catalyzes a reaction that produces a blue pigment, while the recessive allele codes for a defective form of the same enzyme. If you have one or two copies of the dominant allele then you will make functional enzyme and make blue pigment: it is only if you have two copies of the recessive allele that no functional enzyme is produced and no pigment is made.
So, why does a heterozygote look the same as a homozygous dominant in most cases, instead of only half the pigment being present? The answer varies, but usually the cells will continue to make the enzyme until a certain concentration is present, and this will end up the same whether there are one or two copies of the dominant allele

Answer 2

The cell generally speaking expresses both alleles. It's just that due to the function of the individual proteins one has a dominate phenotype to the other. In the case of pigment to no pigment, it's possible that the recessive allele makes no protein at all (a mutation prevents trasncription), or it makes a non-functional protein (protein is made but does not activate any pathways, essentially just sits there) or makes a protein that does it's thing but just doesn't result in pigment. The other dominate allele, however, makes the protein and it synthesizes pigment, so the overall phenotype is one of pigment.

Think of the alleles like two painters, one has various reasons why it doesn't paint the wall, in one case his roller has no paint on it, but he still rolls the wall. In another case he doesn't have a roller. But the other painter present in the room comes along and rolls the wall with paint. Even though you have two painters, you see the result of a painted wall from the dominant allele, or the painter with the paint.

Answer 3

it doesn't. Both alleles are expressed, its just that the expression of one dominates the phenotype.

simplest example would be an allele that encodes a colorless version of a protein for a flower petal, and another allele that encodes a pink version. if the colored allele is expressed, the flower will be pink, since pink will dominae over no colour..

Probably a bit too complex for the question you asked, but there are cases in which a defective allele (usually recessive) is switched off in cells. This is done through recognition of misfolded proteins, and the RNA that makes it. A special pathway then finds the DNA that is responsible for the defective RNA and silences it by wrapping the DNA tightly (thereby inhibiting access) to that allele.