What causes some genetic conditions to be rarer than others?
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Well, there are a lot of factors.
* Dynamics of mutation: some mutations can happen more easily than others. (For example, the mutation that causes Huntington’s Disease happens in a stretch of repetitive DNA that is supposed to read `CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG`. Because of the repetition, the copying enzyme tends to lose its place, making this an unusually unstable part of the genome.)
* Dynamics of evolution: some genetic conditions are going to interfere with reproduction more than others. (For example, since Tay-Sachs Disease causes death in early childhood, sufferers of it will never reproduce. On the other hand, since it’s a recessive condition rather than a dominant one, carriers of it *can* reproduce. Compare with a dominant condition, or with one that has milder and/or later-onset symptoms.)
* Random chance: if a carrier of a genetic condition happened to have a lot of descendants at some point in history, the condition could end up being more common than we would otherwise expect. (For example, Hemophilia B famously spread through the European royalty. It’s easy for rich and powerful men to have a lot of children, even if those men are sickly.)
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