What makes a gene dominant or recessive?

Question

We all carry two copies of each gene (outside of male sex chromosomes). If the two differ from each other often one is dominant and one recessive. How does this mechanism work on a molecular level? What mechanism determines which gene gets expressed?

Answer 1

Thanks for the question! I'll start by addressing it at the level you are raising, but I do want to emphasize that though we generally teach two alleles (one dominant and one recessive) it can actually be more complicated than that (multiple alleles, codominance, incomplete dominance, etc...)

However, given that we are going to move forward with the example of one gene, two alleles (one dominant and one recessive), Iet's keep that as the assumption.

I would encourage you not to think of the dominant gene as the expressed gene and the recessive as not. Both genes are almost always expressed, but the products determine the outcome. If a recessive gene makes less of a protein than the "normal gene" which we will say is dominant in this case, then an effect may not be noticed with only one copy of the recessive gene (Aa), but if two copies of the recessive allele are present (aa) then there may be an insufficient amount of protein produced (such that there is an effect observed). An example of this would be thalassemia (an inadequate amount of hemoglobin in the blood). Usually an effect is only observed in the homozygous recessive individuals. The threshold for observing a recessive disorder is the presence of defects in both alleles (either amount or function). For dominant disorders, any loss or mutation is sufficient to observe a phenotype (there is a higher threshold) OR there is a physical mutation that is acting in a dominant negative way (i.e. the proteins from the alleles form dimers and there is a mutation such that the product of one of the alleles binds with the other product and makes it ineffective. An example here would be fibrillin production in the dominant disease, Marfan syndrome.

So, bottom line, mutations can (but don't always) impact the expression of an allele (if it is in the promotor, for example). Most often, dominant and recessive genes are both expressed -- it is just the variation in product that creates the phenotype.

Feel free to follow up if you'd like further clarification!

Take care,

Heidi