Thursday, September 7, 2017

Endogamy, A Closer Look (Part 1)

[Edited based on comments to include numbers from the Shared Centimorgan Project.]
Subsequent posts are at Part 2 and Part 3.

I've blogged before about how endogamy impacts my matches with known relatives--to hopefully allow for others to extrapolate how it will impact their own genetic genealogy analysis.  But I realized that I have a lot more data--many of my relatives who have tested are have known relationships to one another.  I currently have 355 pairs of relatives (all with at least some Ashkenazic Jewish ancestry) who are related to one another, and I can examine how much DNA each pair shares.

For each relationship section, I give the expected amount of shared centimorgans according to ISOGG.  I've also added in low/high ranges (at the 99th percentile) and average amounts shared according to the Shared Centimorgan Project (SCP).  In addition to looking at overall shared DNA for each pair, I also note the size of the longest shared segment.  Each relationship section gives the average amount of shared DNA and largest segment looking at those who have 100% Ashkenazic Jewish ancestry.

Everyone here has 100% Ashkenazic Jewish ancestry unless noted otherwise in the "Comments" column.  All numbers are as given by GedMatch with the exception of comparisons with Benjamin, JudithB, Shannon & Layla who are only on FamilyTreeDNA.
Marriage Within the Vizel Family (as viewed on

Parent/Child (3400cM expected ISOGG; 3330-3720/3487 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
My father / Me3585.8214.5
My grandmother / Sid3587203.4
My grandmother / My father3587263.7
My mother / Me3587.1281.5
Susie / My grandmother3587281.5
Layla / Jonathan3382267
Shannon / Liz3384267Liz 1/4 AJ; Shannon 1/8 AJ
DaveSr / Dave3585.4281.5DaveSr 1/2 AJ; Dave 1/4 AJ
JudithB / Jonathan3384267
Average (for 100% AJ) 3528.56cM 254.09cM

Siblings (2550cM expected ISOGG; 2209-3384/2629 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
PaulaB / Ina2971.7281.5
DaveSr / Beth2778.5195.3DaveSr & Beth 1/2 AJ
My father / Sid2869.9174.2
My mother / Ed2699.1194.1
Susie / Sid2806.6227
Susie / My father2988.4246.9
Berly / PaulaS2738177.9
Average (for 100% AJ) 2845.62cM 216.93cM

Grandparent/Grandchild (1700cM expected ISOGG; 1156-2311/1766 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
My grandmother / Me1436.4154.8
Average (for 100% AJ) 1436.4cM 154.8cM

Aunt or Uncle / Niece or Nephew (1700cM expected ISOGG; 1349-2175/1750 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Ruth / Ed182590.1
Ruth / My mother1858.3145.2
Ruth / Scott1679115.3
Dave / Beth1687.4130.4Beth 1/2 AJ; Dave 1/4 AJ
Esther / Jonathan1889.698.4
Susie / Me2303223.7
Liz / Beth1762.6125.7Beth 1/2 AJ; Liz 1/4 AJ
Ed / Me1975.6147.8
Sid / Me2028.2196.1
Sara / Myron1865.4133.8
DaveSr / Liz1735.8137.8DaveSr 1/2 AJ; Liz 1/4 AJ
Average (for 100% AJ) 1928.01cM 143.8cM

Great-Aunt or Great-Uncle / Great-Niece or Great-Nephew (850cM expected ISOGG; 251-2108/910 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Ruth / Me1042.3108.8
Shannon / Beth72385Beth 1/2 AJ; Shannon 1/8 AJ
Shannon / DaveSr61484DaveSr 1/2 AJ; Shannon 1/8 AJ
Average (for 100% AJ) 1042.3cM 108.8cM

First Cousins (850cM expected ISOGG; 553-1225/874 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Scott / Ed782.762.7
My mother / Scott1044.276.4
Mitzi / George758.859.6
Leon / JudithK918.661.1
Karen / Ed1071.489.1
Karen / My mother875.868
Liz / Dave908.275.2Liz & Dave 1/4 AJ
Jerry / JudithK887.959.1
Jerry / Leon924.950.2
PaulaS / Sid902.491.4
PaulaS / My father785.961.6
PaulaS / Susie892.797.5
Berly / Sid815.773.3
Berly / My father799.570.1
Berly / Susie838.7106
Myra / Ken815.674.2
PattyBG / Debra861.480.2PattyBJ & Debra 1/4 AJ
Marilyn / Myron1121.8117.7
Average (for 100% AJ) 889.78cM 76.125cM

Half First Cousins (425cM expected ISOGG; 137-856/457 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
George / Ina841.2135.2
Mitzi / Ina450.632.0
PaulaB / George824.2150.7
PaulaB / Mitzi534.468.6
Average (for 100% AJ) 662.6cM 96.625cM

First Cousins Once Removed (425cM expected ISOGG; 141-851/439 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Scott / Me68963
George / Sid42180.5
George / My father504.949.4
Susie / George281.656.6
JudithK / Ruth539.546.7
Mitzi / Sid584.954.7
Mitzi / My father423.154.7
Mitzi / Susie456.142.9
Leon / Ruth584.664.1
Fern / Ruth371.853.9
Karen / Me477.743.3
Jerry / Ruth398.237.5
Amy / Ina660.987
PaulaS / Me497.961.6
PaulaS / George691.2101.7
PaulaS / Mitzi604.741.8
Berly / Me521.269.9
Berly / George469.653.8
Berly / Mitzi470.944.8
Don / George413.367.5
PaulaB / Amy710.787.1
Debra / Beth309.783.8Beth 1/2 AJ; Debra 1/4 AJ
Debra / DaveSr376.267.3DaveSr 1/2 AJ; Debra 1/4 AJ
PattyBG / Beth314.736Beth 1/2 AJ; Debra 1/4 AJ
PattyBG / DaveSr450.965.2DaveSr 1/2 AJ; PattyBG 1/4 AJ
Elise / Hale659.1105.2
Marilyn / Sara574.971.5
Shannon / Dave27939Dave 1/4 AJ; Shannon 1/8 AJ
Average (for 100% AJ) 522.03cM 62.57cM

3/4 First Cousins Once Removed [descended from a man married to two sisters]

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Don / Ina459.652.3
PaulaB / Don531.983.3
Average (for 100% AJ) 495.75cM 67.8cM

Half First Cousins Once Removed (212.5cM expected ISOGG; 57-530/226 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Ina / Sid305.282.9
Ina / My father319.747.2
Susie / Ina334.777.9
Amy / George309.286.9
Amy / Mitzi188.661.1
PaulaS / Ina362.646.1
Berly / Ina231.434.7
Don / Mitzi222.643.1
PaulaB / Sid317.182.3
PaulaB / My father260.774.6
PaulaB / Susie310.574.6
PaulaB / PaulaS379.8114.6
PaulaB / Berly384.657.3
Average (for 100% AJ) 296.67cM 67.95cM

First Cousins Twice Removed (212.5cM expected ISOGG; 43-531/229 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
George / Me269.241.3
Mitzi / Me308.841.3
Average (for 100% AJ) 289cM 41.3cM

Half First Cousins Twice Removed (106.25cM expected ISOGG; 37-360/145 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Ina / Me18075.5
PaulaB / Me16274.3
Average (for 100% AJ) 171cM 74.9cM

First Cousins Three Times Removed (106.25cM expected ISOGG; 0-283/123 SCP)

Pair of relatives Total cM shared (GedMatch) Longest segment in cM Comments
Shannon / Debra10126Debra 1/4 AJ; Shannon 1/8 AJ
Shannon / PattyBG7218PattyBG 1/4 AJ; Shannon 1/8 AJ
Average (for 100% AJ) n/a n/a

Even for these close relationships, particularly for those with 100% Ashkenazic Jewish ancestry, you can see that most of the time the actual shared DNA is higher than expected.  As we look at slightly further relationships in upcoming posts, it'll (in general) get even more skewed.

Stay tuned for additional posts which will look at more distant relationships.  Lots more data to come!  [Part 2 is now posted here.]

Note:  I'm on Twitter.  Follow me (@larasgenealogy).

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  1. Fabulous work!. Another thing I noticed from your data is that the longest segment numbers are also higher than I would expect. Can't wait for the next post!

  2. Lara: Great stuff. Must have taken you a long time to put this together. If you're still totalling things up for parts 2 and later, and if it's not too much extra work, it would be worthwhile to add number of segments as well. Also, I'm sure you have many of these people at FamilyTreeDNA as well, and the numbers there would be useful for comparison.

    For each person, you're only listing one relationship. I'm sure you know some people in this list who are related in multiple ways. Are you just showing the closest relationship? If so, you should add on to their expected for their other relationships.

    Also, I don't understand why there are two groups of parent/child relations. You have 3 ranging from 3382 to 3384 and the other 6 ranging from 3584.5 to 3587.1. Why would they be in two tight groups that are 200 apart?

    1. Will the help of some Excel automation, it wasn't awful. Addressing some of your questions:

      1) I'm already done the later parts--and number of segments would mean doing 355 one-to-one comparisons. For this, I could do a one-to-many and get the data that's here. So maybe in the future....

      2) Yes, I could do FTDNA numbers in the future as well.

      3) All of these people have one known relationship. In the next posts, there will be people with more than one relationship, and they'll be spelled out.

      4) For the parent/child, I'm not sure! Interesting....

    2. Maybe the parent-child gap is because of the company they tested with.

    3. I'd thought that until I realized that Shannon and Liz both tested with FTDNA as did the 3500 range people.

  3. Fascinating! And very helpful. Is it fair to say that the more distant the relationship, the greater the disparity between the expected cM and the average AJ cM? I look forward to seeing the next posts.

    1. Mostly but not always. Stay tuned; lots of good data to come.

  4. This is a great tool. Thanks for sharing your data.

  5. For 3/4 First Cousins Once Removed the expected amount is 265.5 cM (212.5 as half-first cousins once removed via the grandfather plus 53 as half-second cousins once-removed through the grandmothers)

  6. If you are using Gedmatch the expected value for parent-child is 3587 cM, and all the others are repectively larger than you have given.

  7. Thanks for doing this. It would be great to assemble a larger database of known AJ DNA matches to create cousin estimates that take endogamy into account.

  8. This is terrific data. I am looking forward to seeing the larger data set from many families that you are assembling (it is not available yet, right?). Thanks, Paul

  9. I have a cousin that shares 166 cm with an uncle. I would think they were 3rd cousins but researching their tree I have come close to ruling that out. Is it possible they are 4th cousins? So far my Askenazi cousins for which I know their exact relationship have fallen within expected bounds if not a little on the low side. For example I have one 3rd cousin that is only 40cm for me and 151cm for my uncle. Well within range for 3rd cousin and 2nd cousin once removed. This guy with the 166 is a real stumper however. I am into the 2nd great great parent range in his tree and don't see common surnames.