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Frequently Asked Questions (FAQ)
Regarding the Fridig Surname DNA Project
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Question: 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?
Answer: 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.
This additional information is found at http://nitro.biosci.arizona.edu/ftdna/tmrca.html
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Question: How do we arrive at the
mutation rate assumption (.002 or .004) for our genealogical DNA
studies? Answer: It is very likely that each of the
Y-chromosome microsatellite markers have different mutation rates. Since the
current database of specific marker information is so small statistically, we
are required to assume a standard mutation rate for all microsatellite markers
until such time reliable individual mutation rates can be constructed for
each marker. Currently the most accepted or standard rate is 1/500 =
0.002 which is the average pedigree rate across three significant reported studies--Heyer et
al (1997), Bianchi et al (1998), and Kayser et al (2000). Some
researchers choose to use a higher mutation rate of 1/250 = 0.004 which is
considered by most geneticists as likely to underestimate (too short) the true
times for the most recent common ancestor. This higher mutation rate
is based on the reported study of Holtkemper et al (2001).
This Page Was Last
Updated:
06/07/2005
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