- So what is a mutation and why do they happen?
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.
When these mutations occur in the 'junk' region of DNA, the cell still remains to work perfectly normally.
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.
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.
- How rare is rare?
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.
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)
If we take a look at the three lineage diagrams below, we can see several examples in which 10 transmission events can occur.
Several paths to 10 'transmission events' (only males are shown)
http://www.dnaheritage.com/images/ma...n_ancestor.gif
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.
(Maths: 21 markers x 0.002 x 10 transmission events = 0.42 mutations)
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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