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Mutations and mutation rates |
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- So what is a mutation and why do they happen? |
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A mutation is usually defined as a change in the DNA sequence
and can occur due to several different reasons, which include
diseases, radiation and chemicals. However, with STR’s
(short tandem repeats), as are tested in the Y-DNA test, these
changes are thought to be caused by the miscopying of the DNA
strand by a certain enzyme. |
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When these mutations occur in the 'junk' region of DNA, the
cell still remains to work perfectly normally. |
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When cells in our body divide through mitosis, the DNA has
to make a copy of itself to give to the new cell. It does this
by using an enzyme called DNA polymerase. Using the original
DNA strand as a template, the DNA polymerase works its way along
the strand reading the original code. The DNA polymerase then
uses other bases nearby to build a completely new strand of
DNA. |
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At some locations along the DNA strand, the code repeats itself.
These are our short tandem repeats. Occasionally, when the DNA
polymerase reaches this point, it gets confused and causes a
‘slippage’. Instead of reading and faithfully reproducing
13 repeats, for example, the DNA polymerase produces 14 repeats.
So this is how repeats can go up or down. It is a rare occurrence,
but this can of course give rise to a difference in repeat numbers
between father and son. |
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- How rare is rare? |
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The polymerase that copies DNA is pretty good at what it does
and at any single STR location, it is estimated that a mutation
will occur only once every 500 'transmission-events' –
or roughly 0.2% per generation. Basically, a transmission event
is the birth of a baby boy, but it is also an event where a
mutation can occur and be passed on. The rate of this genetic
clock is still under debate – some STR’s will change
more rapidly than others and more research needs to be done
in that area, but overall 0.2% per generation is a good working
estimate. |
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If we increase the number of markers we test, the observed
number of mutations will go up accordingly. Using 21 markers,
we can expect to see a mutation once every 24 transmission events.
(Maths: 500 / 21 markers = ~24)
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If we take a look at the three lineage diagrams below, we
can see several examples in which 10 transmission events can
occur. |
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Taking the top lineage only (i.e. 10 sons), if we were to
test every individual we would only expect to see 0.4 mutations.
This is less than one, so we therefore would not expect to see
a single mutation at all. |
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(Maths: 21 markers x 0.002 x 10 transmission events = 0.42
mutations) |
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The chance of a mutation occurring is the same whichever lineage
you choose above, as they all have 10 transmission events. |
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