Is further testing needed to determine half sibling?

Question

On a DNA match I have a 'close relative' that came up matching 2039 Cms across 58 segments.

Do we need to do more testing to determine if we are definitively half siblings?

Answer 1

Additional tests of the two people involved themselves aren't going to do much, although a bit more analysis of your existing results can resolve one possible ambiguity.

Using this Shared cM Project tool shows that a 2039 cM match has a 6% chance of being a full sibling relationship. This possibility can be eliminated or confirmed by looking for full-identical segments among the half-identical matching segments. If the test results are both on 23andme.com or both on GedMatch.com, you can view this information. (The results can be uploaded to the free GedMatch.com from all the major testing services.) Full siblings have full segments as well as half matched segments, while half siblings do not have fully matching segments (other than random tiny bits). More on this can be found on the ISOGG's "fully identical regions" page.

You should be aware that the most likely result for a 2039 cM match includes not only half-sibling as a possible relationship, but also others: Grandparent, Grandchild, Aunt/Uncle, or Niece/Nephew. These cannot be distinguished from just the DNA results of the two principle people. Additional knowledge of age and circumstances can guide the elimination of alternatives, as can additional DNA testing of other likely relatives, such as testing of a candidate for the common parent.