I know that most eyes looking at the emails from this list probably glaze
over when the subject of genetics comes up. For those interested, here's
an explanation of a couple of terms that might help.
SNP, which stands for Single Nucleotide Polymorphism:
The "alphabet soup" of DNA is comprised of those chemicals that abbreviate
to A, G, C and T. They're called nucleotides. There are more nucleotides
but only those four make up the molecules of DNA. Occasionally a SNP will
mutate from one of those four letters to another. For example, the SNP
we've talked so much about, L448, nicknamed "Young Scandinavian," is the
result of a C becoming a T--a simple chemical conversion. This is believed
to have happened with an individual in Norway about 2300 years ago. SNPs
tend to be highly stable. Once a mutation occurs, it is passed down
unchanged for hundreds or thousands of generations. I had about 1700 SNPs
sequenced at 23andMe. Don had very many more sequenced at NatGeo.
STR, which stands for short tandem repeat:
This is a different animal. The results we see at
http://www.familytreedna.com/public/Cooley/default.aspx?section=yresults
are STRs. They represent a string of nucleotides (A, T, C and G) that
repeat a certain number of times. For example, DYS393--the first column
from our results page, has the sequence AGAT repeated X number of times.
In our case, it's repeated 13 times. Someone who has 15 repeats at that
location, as is the case with the testers in group CF12, are said to be a
genetic distance of two from us at that position. Unlike SNPs, STRs are
volatile and regularly add or delete a sequence or more with some
regularity.
The stability of SNPs, then, are great for "deep ancestry," the sort of
thing that population geneticists are interested in. STRs, on the other
hand, are great for genealogy because the nature of their volatility means
that comparisons can be made within about 15 generations, instead of
millenia. In other words, two patrilineages from the same individual 20
generations ago, say about 650 years, are not expected to be easily
matched--too many STR mutations would have occurred along each line.
Feel free to ask questions.
-Michael
--
Second VP, the Cooley Family Association of America
Administrator, the Akins DNA Project
Administrator, the Ashenhurst DNA Project
Administrator, the Bishop DNA Project
Administrator, the Eldridge DNA Project
Administrator, the alt-McDowell DNA Project
Co-Administrator, the Cooley DNA Project
Co-Administrator, the McDougall DNA Project
Instructor "Genealogy and Family History," the Osher Lifelong Learning
Institute (OLLI)
B.A. Humboldt State University, History
Received on Sun Nov 03 2013 - 23:25:30 MST