How can I be certain how closely I actually relate to my cousin matches when I know that I have a person in my tree who is my 4th Great Aunt on my Father's Maternal side. This same person is possibly my 4th Great Grandmother on my Father's Paternal side. How would endogamy affect my matches to her descendants?
How much more closely would we appear to match?
The number of matching segments and total cM work in an additive manner.
If, for example, you had 3 segments totaling 25 cM you share with your 4th Great Aunt through your father's maternal side, and 2 segments totaling 20 cM to her through your father's paternal side, then in total you would share 5 segments totaling 45 cM with your Great Aunt.
Since 1 generation averages 1/4 the total cM of the previous, a relative who is equally related through two different lines would appear to be a half a generation closer than they are via either line individually.
Remember, of course, that in endogamous populations there may be other unidentified connections as well. Maybe your 4th Great Aunt is also connected through 4 other ancestors and each has just 1 matching segment of 12 cM, 10 cM, 8 cM and 8 cM. Then in total, your 4th Great Aunt would match with you on 9 segments totaling 83 cM and would appear to be a whole generation closer than just one of the identified matches indicate.
Because of this, endogamous populations have different cM ranges for the identified relationship than non-endogamous populations do. Lara Diamond has done a survey to determine ranges. Her results are available at her Ashkenazic Shared DNA Survey.
Lara has also compared how closely her relatives match the theoretical ranges in her articles: Endogamy, A Closer Look (Part 1), Part 2 and Part 3.
