Put another way, someone who shares 1315 cM with you could be in either group (and remember that each group includes multiple possible relationships). Notice another problem? The low end of the Group B range overlaps the high end of the Group C range. Group C can range from 575 cM to 1330 cM. That is, someone in Group B might share 1750 cM with you, but they could also share as little as 1300 cM or as much as 2300 cM, according to the DNA Detectives chart. To complicate matters, each group is defined not so much by an average or “expected” amount of shared DNA but by a range. The original chart is available in the files of the DNA Detectives Facebook group. Shared centimorgan ranges for different relationship groups. I will use the DNA Detectives group names in the rest of this post for ease of reference. In the two examples above, grandparent/child, aunt/uncle, and half sibling would be Group B, and great-grandparent/grandchild, first cousin, great-uncle/aunt/nephew/niece, or half-uncle/aunt/nephew/niece would be Group C. The DNA Detectives Facebook team has designed a nifty chart that categorizes relationships into groups based on the expected amounts of shared DNA. (In this case, you can narrow the possibilities using age.) Someone sharing 950 cM with you could be a great-grandparent/grandchild, first cousin, great-uncle/aunt/nephew/niece, or half-uncle/aunt/nephew/niece. Those relationships are indistinguishable based solely on the amount of shared DNA. For example, a woman who shares 1750 cM with you could be your grandmother, granddaughter, aunt, or half sister. Why not? One reason is that multiple different relationships can give the same patterns of shared DNA. Unfortunately, with the exceptions of identical-twin, parent–child and full-sibling matches, that’s simply not possible. The one thing we genealogists probably want most from our autosomal DNA matches is something they can’t give us: an exact relationship prediction based on shared DNA alone.
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