From Kimberly Powell,
Your Guide to Genealogy.
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Understanding the Results of Your Y-Chromosome DNA Test (Y-Line):
Your DNA sample will be tested at a number of different data points called loci or markers and analyzed for the number of repeats at each of those locations. These repeats are known as STRs (Short Tandem Repeats). These special markers are given names like DYS391 or DYS455. Each of the numbers that you get back in your Y-chromosome test result refers to how many times a pattern is repeated at one of those markers. The number of repeats is referred to by geneticists as the alleles of a marker.

What is the effect of adding more markers?
Adding additional markers increases the precision of DNA test results, providing a greater degree of probability that a MRCA (most recent common ancestor) can be identified within a lower number of generations. For example, if two individuals match exactly at all loci in a 12 marker test, there is a 50% probability of a MRCA within the last 14 generations. If they exactly match at all loci in a 21 marker test, there is a 50% probability of a MRCA within the last 8 generations. There is a fairly dramatic improvement in going from 12 to 21 or 25 markers but, after that point, the precision starts to level off making the expense of testing additional markers less useful. Some companies offer more precise tests such as 37 markers or even 67 markers.

Understanding the Results of Your Mitochondrial DNA Test (mtDNA):
Your mtDNA will be tested on a sequence of two separate regions on your mtDNA inherited from your mother. The first region is called Hyper-Variable Region 1 (HVR-1 or HVS-I) and sequences 470 nucleotides (positions 16100 through 16569). The second region is called Hyper-Variable Region 2 (HVR-2 or HVS-II) and sequences 290 nucleotides (positions 1 though 290). This DNA sequence is then compared to a reference sequence, the Cambridge Reference Sequence, and any differences are reported.

The two most interesting uses of mtDNA sequences are comparing your results with others and determining your haplogroup. An exact match between two individuals indicates that they share a common ancestor, but because mtDNA mutates extremely slowly this common ancestor could have lived thousands of years ago. Matches which are similar are further classified into broad groups, known as haplogroups. A mtDNA test will provide you with information about your specific haplogroup which may provide information on distant family origins and ethnic backgrounds.

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