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I am a female. My half paternal brother and I share 328.7 centimorgans across 13 dna segments.

Is that possible, or should the numbers be higher?

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At 328.7 cM, you're not half-siblings.

According to the Shared cM Project for 328 cM, the two highest probable sets of relationships are:

  • 48.46%:

    • Half GG-Aunt/Uncle
    • 2C
    • Half 1C1R
    • 1C2R
    • Half GG-Niece/Nephew
  • 44.99%:

    • Great-Great-Aunt/Uncle
    • Half Great-Aunt/Uncle
    • Half 1C
    • 1C1R
    • Half Great-Niece/Nephew
    • Great-Great-Niece/Nephew
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Are you sure that 328.7 cM is the total match, and not the size of the largest segment? Some websites, such as GEDmatch, report both numbers.

For example, on GEDmatch, my full brother and I share a total cM of 2911.7, and our largest match is 171.3 .

Screen clipping from GEDmatch

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  • 328 cM being a single segment is very unlikely, as the longest cM length used for any chromosome is about 284 cM. – RobertShaw Aug 13 '18 at 17:46

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