system
Given the following post, respond as if you're commenting on Reddit:
human
If genetic mutations are random, why are some (nonhereditary) mutations so common while others are rare?

For example, why do some types of cancer account for the majority of cases while others are rare? What is stopping more people from getting those rare cancers more often? I know environmental influences + cell type + other genes/genetic predispositions are definitely big factors, but it almost feels like a paradox to just call all mutations random, even though I know that to be true. 

To clarify, I'm not trying to challenge the idea that genetic mutations are random, I'm just trying to understand the framework, especially in the broader context of evolution.



Edit: Thanks everyone who's responded! This has pointed me in the direction of thinking more along the lines of distribution of cancers + mutations in general in different types of cells + in specific genes and considering the complex environmental causes/risk factors (?) of cell mutations, which feels more comprehensive and like I'm on the right track. I feel like I may need to go more in depth to understand this in the broad context of genetics + evolution, so I might have to look for some good (maybe basic, for now) reading materials or something.
gpt
Just adding this: although genetic mutations are “random” in the sense that whether they happen or not in an individual is, effectively, a roll of the dice, that doesn’t mean the dice are equally likely to land on all numbers.  For example, it’s known that one nucleobase (cytosine, C) has an outsized tendency to mutate to thymine (T) because it only has to lose one amino group to turn into uracil, which looks to many enzymes indistinguishable from T.  This even happens when you heat DNA in a test tube - a very small fraction of the C’s in that test tube will lose an amino group and become U.   (Edit: and if the C was already methylated, it will turn into T if it loses the amino group.)  This is actually a nuisance when you are looking for rare mutations in a sample, and can lead to false positives unless you correct for it somehow.

There are many other examples, but even at a chemical level certain DNA bases and sequences are more susceptible to mutation than others.  This is at least part of the answer to your question.

Edit: So, while the occurrence of a mutation or not in an individual can be considered an essentially random process in most cases, not all random mutations are equally likely.  It’s like if you had a hat full of names and drew a name at random: the likelihood that the name begins with S isn’t the same that it begins with X, just because S names tend to be more common (at least in English).  The process can be random and still generate certain outcomes more than others.