Is a 1000 CM match on an Ancestry autosomal test unusually high?

+3 votes
I have three first cousins who DNA tested with Ancestry, one who showed a match of 1006 cM's. The other two were 760 and 590. I know matches can vary widely, but I wondered why one was so much higher, and then I did her tree, and some surnames back a few generations were the same as my mother's mother. Could the 1006 cM's be so much higher because of that--or is it still within the normal boundaries?
WikiTree profile: Bob Scrivens
in The Tree House by Bob Scrivens G2G6 Mach 1 (19.3k points)
retagged by Bob Scrivens

My Ancestry results for 1st Cuz’ns are:

Paternal line:

  1. 1,095 cM across 45 segments
  2. 1,042 cM across 51 segments
  3. 818 cM across 35 segments
Note:  1. and 2. above are siblings 

Average: 874 cM

High - Low: 1225 - 553 cM

Source (see chart in item 2)

Hi Bob, it's not high for a 1st cousin, but endogamy can easily increase the amount of mutual DNA and cM count in your matches where you might have double, triple or more cousinship. Very common in Acadian descent.
One thing I just looked at was ethnicity.

What I saw was the closer my cousins and I shared the same percentage of ethnicity the higher the cM.
Please add the dna tag to your question.
Good idea, Julie. Did it.

2 Answers

+5 votes
Best answer
The 15 AncestryDNA cases I have vary from 606cM to 966cM; average 816cM and standard deviation of 120cM.

So while 1006cM is outside of what I've seen (by a little bit), it's (1006cM-816cM)/120cM = 1.58 standard deviations above the average. For a normal distribution, 90% of the values should be within 1.64 standard deviations.

In other words, it's within the bounds of what you'd expect to see 90% of the time - kind of high, but not all that unusual.

Also, notice that the average of your three is 785cM, really not very far from my average.

Intermarrying will certainly increase the numbers, but if you went back three generations and still didn't see a common ancestor it's not going to amount to anything very significant.

It could just be that the two of you simply happened to get a somewhat above-average fraction of your genes from the same grandparent.

The shared cM project chart is notorious for having overly broad ranges, BTW. It lumps several testing companies together (which is bad, because they all do things differently, and the results come out a bit different), he averages in samples reported to have endogamy (which is just incompetent), etc. For 1C on AncestryDNA, the 99% interval should be something in the low 500cMs to something in the low 1100cMs.
by Frank Stanley G2G6 Mach 6 (67.0k points)
selected by Bob Scrivens
+6 votes
by Leandra Ford G2G6 Pilot (119k points)

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