"Alfred Einstead"  wrote in message
news:ccd7e4$8uk$1{at}darwin.ediacara.org...
> Ron Okimoto  wrote:
> > > Looking at a DNA database, I notice that the patterns of
> > > mutations, insertions and deletions are extremely non-random.
> > > ... By far, most of all
> > > changes are of one type (between C and T), and most of the
> > > remaining, by far, are of another type (between A and G).
> > >
> > > Is this accidental, or is it something that's generally true?
>
> > The biases noted so far have a basis in DNA chemistry.
> > Transition (pyrimidine to pyrimidine or purine to purine:
> > T to C, C to T, A to G, and G to A) mutations are much
> > more common than transversion (purine to pyrimidine like,
> > T to A) mutations.
>
> That's understood.  But what's not understood is the
> prevalence of CT over AG; somewhere between a 5:1
> to 10:1 ratio.

At the risk of pointing out the obvious, a CT mutation
on one strand is almost certainly matched by an AG
mutation on the other strand.  But only one strand, the
"interesting" strand, appears in your database.  That
interesting strand is under selection.  It is not shocking
that an CT change in a control sequence might be
selectively neutral, whereas an AG change is
likely to have an adverse functional effect.

So, perhaps your title should read "is selection actually
random?".  And the answer is: "Of course not."
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