What exactly determines if a gene is dominant and which isn’t?

Question

In the title. Certain genes and traits are dominant, some are not. Was causes some genes to be dominant over others?

In: 1

Answer 1

I own two box graters. I can zest a lemon or grate some parmesan on either one.

If one of my box graters were to break, I’d still have the other and would still be happily zesting lemons and grating parmesan. Someone observing my behavior or “output” would not notice any difference. It’s only if the second box grater breaks too that things change. Now I have no functioning box graters, and so no more lemon zest or grated parmesan.

Let’s translate this to genes. I have two copies of the “box grater gene”. Each copy can be one of two versions – in genetics we call these “alleles”. The two versions are “working box grater” and “broken box grater”. WBG and BBG, for short. If I have at least one WBG copy, then I am able to grate things. Doesn’t matter if the other gene copy is BBG. If I have two WBG’s, more power to me but it doesn’t make a practical difference. Genetically then, we would say that the WBG allele is *dominant* while the BBG allele is *recessive*.

Note that this is all rather simplified. Dominance is typically not a binary thing. Maybe people who have two box graters are able to produce a bit more lemon zest and grated parmesan, because they don’t need to pause when one of their box graters needs cleaning. Or maybe there’s a third allele called “half box grater”, where you need to copies of this HBG allele to make one full, functioning box grater. In that case the BBG allele is actually dominant over the HBG, because having even one BBG is enough to make the HBG copy useless. And so on.