What makes a gene either strong or weak, in terms of expression and chance/s of inheritance? Why are some genes more likely to be passed on than others?
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Unfortunately, genetics is fiendishly complicated and there isn’t a very simple answer to this question, because there are many different pathways by which genes can be expressed (or, not expressed.) Part of the reason this is is because genes don’t encode traits directly. Each gene is rather just instructions that tells cells to make a certain protein with a certain shape. That’s it. It’s the complex and interconnected system of proteins all interacting which gives rise to physical traits.
But let’s take a relatively simple example: a gene that encodes for a protein that happens to not do anything. The protein in question should do some function, but the way that this gene instructs cells to make it, it doesn’t work. Now consider another gene that has instructions for a working version of this protein. These two different genes might exhibit the familiar dominant-recessive relationship that you might have learned about in school. This is because everyone gets two versions (called alleles) of most of their genes, one from their mother and one from their father. If you have at least one allele that makes the working protein, you’ll end up with some of the working protein being made in the body. You need to have two copies of the “broken” protein gene in order to end up with none of it. And if only a little of the working protein is necessary to result in some trait being expressed, then the trait can be “dominant,” because only one copy of the gene is plenty to get enough of that protein.
That’s a very simplified example. Many traits are actually controlled by many different genes making many different proteins, so the dominant/recessive relationship is not so clear-cut.
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