Understanding Genetic Distance

this page has been mirrored from the FTDNA site

When comparing people’s samples in our system we show individuals who are closely matched, but not identically matched, as being different by what the Anthropologists call genetic distance.

If two people were identical in all markers except they are off in one marker by 1 point, the genetic distance would be 1. If they were off at 2 different markers by 1 point in each marker, then the genetic distance of those two samples would be 2. If they were off by 2 points at one marker and 1 point in a second marker, then the genetic distance would be 3. This is called the Stepwise Model of calculating genetic distance for shallow time depths. (i.e. Genealogy not Anthropology)

An example of a genetic distance of 1 is shown below:

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393	390	19	391	385a	385b	426	388	439	389-1	392	389-2
12	23	14	10	15	17	11	17	12	13	11	29
12	23	14	10	14	17	11	17	12	13	11	29

Some markers have shown themselves to be more volatile than others and the population geneticists have created a second model to account for these ‘aberrations’. That model is called the Infinite allele model. For markers that fall into this category, despite the fact that two people could be separated by 2 (or 3) mutations, the scientific assumption is that the change took place in a single generation (between a father and a son) and therefore it is treated as a single step, despite the fact that more than one ‘point’ separates two samples.

Currently the Scientists have asked us to classify DYS 464 and YCAII a and b as following the Infinite Allele Model.

For more on the models used in the scientific community please see this web site built by Dr. Bruce Walsh, a scientific advisor to Family Tree DNA:

                              http://nitro.biosci.arizona.edu/ftdna/TMRCA.html

Therefore, when you look at your Y-DNA match page and you see a genetic distance of 1 you most likely have a difference from another person as illustrated in the chart above. The same holds true for a difference with a genetic distance of 2…you are off from another person by 2 - single point mutations.

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