In the title. Certain genes and traits are dominant, some are not. Was causes some genes to be dominant over others?
In: 1
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.
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