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"STR markers

These repeating sets of nucleotides are referred to as genetic markers and are designated by a DYS number (DNA Y-chromosome Segment number). Marker values, which range from 8 to 31 in the examples below in Haplotype, are the number of times each segment repeats.
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SNP markers

SNPs are changes to a single nucleotide in a DNA sequence. The relative mutation rate for a SNP is extremely slow. This makes them ideal for marking the history of the human genetic tree. SNPs are named with a letter code and a number. The letter indicates the lab or research team that discovered the SNP. The number indicates the order in which it was discovered. For example M173 is the 173rd SNP documented by the group who uses the letter M. "
Source(s):
http://ask.inwiki.org/Genealogical_DNA_test

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