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I have one as well that is herideraty but no relatives have it (tested after finding out I had it). I think it's usually something like 1 in 400k people having a mutation of a specific gene.

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De novo mutations (DNM), that is new mutations, is rather high and increases stronly with the age of the father, because sperm cells are produced by continued cell divisions, where the egg cells of women are produced before birth. With the mother there is also a slight increase over time, but the number of even old mothers is lower than that of young fathers. Luckily, only a small part of the DNM occur in genes and some mutations are not even harmfull, due to how triplets of base pairs are decoded into amino-acids. And then there are also many mutations that do have an effect on the protein being produced, but that the difference does not have a great effect. For most genes there are many variants, which all leads to working proteins, some of which might work slightly better than others in some area and maybe (slightly) better in another area. (Just recently there was a news item about how humans lack a certain gene compared to other mamals, which increases our risk with respect to cardiovascular disease, but apparently also increases our ability to walk/run long distances.)

> the egg cells of women are produced before birth

just fyi this is actually an active research topic, c.f. https://en.wikipedia.org/wiki/Oogonial_stem_cells#Research

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