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What does Y-STR DNA testing tell us?

Read more about mutation rates 

Put simply, if the markers all match then those tested are very closely related. If a marker does not match, this is known as a mutation, and the more mutations that exist, the further apart those being tested.


As a rule of thumb, in the FTDNA test of 67 different markers, there is a 0.32% chance of any one of the 67 mutating, which means that there is, statistically, the expectation of just one mutation every 100 years for each person tested.

Using the number of mutations and the average expected mutation rate, we can calculate the statistically likely generation gap between 2 parties being tested. But a word of caution on the maths - and more on this later. The calculation to the most recent ancestor rests on the average mutation rate and there is a great deal of debate and confusion on this topic. Early studies (Walsh 2001) indicated a rate of 0.0020, or 0.2% per marker, but later studies has shown mutation rates of 0.0040, or 0.4% per marker, depending on the markers being assessed.

The existence of more recent higher mutation rates is very easy to explain. Walsh studied only slow mutating markers, and the results were valid for these markers. More recent studies have used many more markers than Walsh, and these markers have higher mutation rates, which raises the average.

Between testing companies, the average mutation rates will also vary as different markers are tested. Using accurate mutation rates is key as if the wrong ones are used, for example applying Walsh 0.2% rates to a set of markers which includes faster mutating markers, the estimate of the most common recent ancestor will be wildly out.