If two men have a Y-chromosome match on 25 of 26 markers, but there is a two step difference on one marker, is that considered one or two mutations?
Currently it is perceived that most microsatellite marker mutations appear to be single-steps, but there are rare cases (about 1/30 – 1/50) of two-step mutations. With the current level of genetic thinking, it is not possible to interpret this knowledge at this time with any degree of certainty, but according to Dr. Bruce Walsh, geneticist at the University of Arizona, if all the other markers match between the two individuals, you can treat a two step mutation as a single mutation event. If there are multiple marker mismatches (three or more) that implies that the Most Recent Common Ancestor (MRCA) lived so long ago that there has been time for two independent mutations at that one marker. Dr. Walsh has more background information and a lookup table where you can check the effect if you count the two step mutation as one mutation or two.
