But wouldn't this make Force sensitivity too common? Running the exact numbers isn't actually feasible without making huge, unfounded assumptions about the frequency of the alleles in the population of the Star Wars universe. But moreover, it would still be a futile exercise, because there exist other reasons that could help explain why the galaxy isn't overrun by Force-wielders.
Just like our fictional Force sensitivity "condition," albinism is a Mendelian trait that is caused by two recessive alleles. The worldwide frequency of albinism is said to be about one in 17,000, but societal factors play a huge role in reducing the frequency of albinos in the population, due to the oftentimes horrible discrimination they face.
While Force users wouldn't experience the same form of discrimination, societal factors will similarly reduce their frequency. The Jedi order is monastic and preaches freedom from emotions, including love, so Jedis don't strike me as the type to go around making babies. This would diminish the frequency of the allele in the population. Add to this the fact that in the past, the Jedi order aggressively recruited Force-sensitive individuals, effectively putting restrictions on their ability to reproduce, and we see a form of self-imposed population control. Furthermore, at the start of Episode 4, the Jedi and Sith are all but wiped out due to their mutual genocide, which further diminishes the frequency of the allele.
But beyond that, it is actually possible to have the allele not be extremely rare while still maintaining a low number of Force users. Luke and Leia didn't know they were Force-sensitive until Jedi pointed it out. Just because someone has the capacity to be Force-sensitive doesn't mean that they automatically know how lift an X-Wing out of a swamp with their mind - or even that they can. This means there could actually be far more individuals with the two recessive alleles than is apparent. Plus, it's clear that some degree of training is required to manipulate the Force. Maybe someone is Force-sensitive, but that doesn't mean they will have the discipline to do anything with that capability. Add to that personality, morals, political views, etc. and you could imagine that the number of people that wouldn't make the cut in the Jedi Academy is fairly significant.
And if you'll allow me an aside to discuss the story, I rather like the idea behind that last paragraph: Force sensitivity isn't all that uncommon. Luke represents the everyman. He's a character that we can all relate to. He's not a high-power political figure like Leia; he's not a mystical wizard like Obi Wan; he's not the dashing rogue that we'd all like to be - he's just a simple guy with a boring job and (seemingly) humble origins who one day discovers that he can be much more than he is. By not making Force sensitivity ridiculously rare, I believe the audience can actually better engage with Luke's story and the feeling of hope it carries - because deep down, there's that childish part of us that wants to believe that with the right training, we too could move objects with our mind.
Now, we've addressed most of our initial bullet points: Force sensitivity is passed on to offspring through genes; individuals who are Force sensitive have the two recessive alleles (Hell, let's call them the midichlorian alleles, because why not?); Force users are rare through a combination of genetic and societal factors. That leaves us with one point: that some people are naturally stronger in the Force than others. Our current model can't quite explain this - you either have Force powers or you don't. However, we can invoke more complex genetics to do so.
Truly Mendelian traits are exceptionally rare. In the past, it was believed that hair and eye color were Mendelian, but it turns out that multiple genes determine what color your eyes and hair are. Similarly, multiple genes may determine the degree to which someone is Force sensitive. An alternative - or coincident - explanation could invoke multiple alleles as well, as with blood type. It is actually possible to have multiple types of dominant or recessive alleles for a given gene, and how they pair up will result in more than two outcomes. We see this with blood type, which has three alleles: type A (dominant), type B (dominant), and type O (recessive). This leads to situations with incomplete dominance, codominance, and more that are beyond the scope of today's discussion. The bottom line is that, if you're willing to get deep enough into genetic complexity, you can account for even greater rarity, varying degrees of Force sensitivity, and ultimately do a much cleaner job of explaining why some people can use the Force while others can't without having to invoke the idea that every single living thing has mystical microbes in its bloodstream.
But hey - we can also just say a space wizard did it.
For more science in Star Wars articles, check out Just How Realistic Are the Single-Biome Planets?, This Is How You Defend The Ridiculous Crossguard Lightsaber, and 4 Science Mistakes Star Wars: Episode VII Needs to Fix.