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doc_nano t1_iztz3dj wrote

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.


FogeltheVogel t1_izx28kn wrote

It's always important to remember that "random" does not mean "equal chance".


doc_nano t1_izx3d04 wrote

Yep, a common misconception. I can’t count the number of times I’ve heard something like:

“Either you’ll get cancer or you won’t, it’s 50/50.”

No, 2 options doesn’t mean it’s a 50% chance of each!