In the title. Certain genes and traits are dominant, some are not. Was causes some genes to be dominant over others?
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.
Genes mostly act by producing proteins with specific structures coded for by the gene.
Dominant genes produce a protein that *does something*. So for example, imagine that apples had a chemical in them that could be metabolized into DeAdlY nEuroTOXin if you had a particular variant of a protein. The normal variant of that protein, obviously, would not do this, since that’s hugely detrimental. But if a mutant variant did do that, the gene for it would probably be dominant. Having any of the “produce the deadly neurotoxin if you eat apples” protein around would make apples deadly poisonous to you, even if you’re also producing the normal variant of the protein as well.
Recessive genes, on the other hand, usually *knock out* the function of a gene. So for example, if you had a protein that lets you digest apples, where the normal variant digests them and the mutant variant does not, the variant for not-digesting-apples would likely be recessive. Because having a single copy that produces the can-digest-apples protein will allow you to digest apples, even if you’re also producing a broken variant.
This isn’t a strict binary, of course. You can have genes where having a mix of the two can produce some slight change but doesn’t give you the full variant. That’s the case with sickle-cell anemia, for example, which is related to a mutant gene for the hemoglobin that makes up most of your red blood cells. It’s a recessive disease, because the cells only bend out of shape if all the hemoglobin is the variant type. Half and half is enough to keep a mostly normal shape to the red blood cell – but it bends very slightly in a way that happens to confer resistance to the malaria parasite. So there’s a spectrum here. In other cases, which variants you have are what matter – that’s the case with blood types, where A and B are two variants of the same protein and AB means you produce both.
But in most cases, half of a normal production of a protein is plenty, since proteins are usually not produced in exactly the needed amount. So most traits coded for by a single gene lean towards one or the other.