How much DNA would match a 5th cousin once removed?

Question

I have 5 autosomal DNA tests (from cousins and siblings of each other) that are theoretically all 5th cousins, once removed, from another DNA test. Using GED Match autosomal compare, I found these matches, all in different chromosomes with:

• minimum threshold size 300 SNPs
• mismatching-bunching limit 150 SNPs

• Minimum segment cM 4.0 cM:

  1. Wilson:
     • Chr 1 120,064,541 to 149,717,828 4.2 cM 822 SNPs
     • Chr 11 130,961,134 to 132,394,867 4.1 cM 546 SNPs
     • Chr 16 28,165 to 1,175,315 4.2 cM 310 SNPs
     • Chr 18 1,799,217 to 3,064,785 4.7 cM 425 SNPs
     • Chr 18 4,128,311 to 5,452,488 4.7 cM 444 SNPs
  2. Meredith (Sister of Wilson):
     • Chr 1 243,385,031 to 245,045,127 4.2 cM 514 SNPs
     • Chr 5 168,169,228 to 169,810,385 4.1 cM 617 SNPs
  3. Carol:
     • Chr 6 14,175,149 to 16,601,341 4.3 cM 672 SNPs
     • Chr 19 53,015,545 to 55,106,529 5.2 cM 618 SNPs
  4. Lawrence:
     • Chr 12 11,649,134 to 12,812,400 4.2 cM 422 SNPs
     • Chr 15 93,109,846 to 94,320,114 4.2 cM 377 SNPs
     • Chr 19 8,492,086 to 9,996,711 4.5 cM 340 SNPs
  5. Clyde (brother of Lawrence):
     • Chr 13 24,623,069 to 26,284,031 4.2 cM 463 SNPs
     • Chr 15 93,109,846 to 94,320,114 4.2 cM 378 SNPs

Do these results look like a 5th cousin once removed relationship? Should I compare with different threshold values? I've already adjusted down the minimum threshold size to 300 SNPs from 400 SNPs.

For reference, here is a compare of believed 3rd cousins using the same parameters (Mark vs. Edward):

• Chr 1: 5.5 cM 324 SNPs
• Chr 7: 23.4 cM 2,273 SNPs
• Chr 8: 14.3 cM 3,218 SNPs
• Chr 20: 5.1 cM 356 SNPs

And here is a believed 3rd cousin once removed compared (Edward vs. Louis):

• Chr 15: 6.6 cM 380 SNPs
• Chr 22: 5.1 cM 316 SNPs

And here is another believed 3rd cousin once removed compared (Mark vs. Louis):

• Chr 10: 7.8 cM 693 SNPs

Here is 3rd cousin once removed (Edward vs. Wilson and Meredith, both children of Mark):

• Wilson vs. Edward:
  • Chr 7 5.8 cM 766 SNPs
  • Chr 7 4.2 cM 445 SNPs
  • Chr 8 14.0 cM 3,126 SNPs
  • Chr 17 4.6 cM 313 SNPs
  • Chr 17 4.5 cM 543 SNPs
  • Chr 18 4.8 cM 427 SNPs
  • Chr 20 5.0 cM 366 SNPs
• Meredith vs. Edward:
  • Chr 7 24.0 cM 2,340 SNPs
  • Chr 8 12.4 cM 2,751 SNPs
  • Chr 17 4.6 cM 314 SNPs
  • Chr 17 4.5 cM 545 SNPs
  • Chr 18 4.8 cM 428 SNPs
  • Chr 20 4.4 cM 304 SNPs
  • Chr 20 4.6 cM 475 SNPs

And here is 4th cousin (Louis vs Wilson and Meredith, both children of Mark)

1. Wilson vs. Louis:
   • Chr 2 4.2 cM 907 SNPs
   • Chr 5 4.4 cM 651 SNPs
   • Chr 10 4.3 cM 340 SNPs
   • Chr 12 4.0 cM 474 SNPs
   • Chr 15 5.0 cM 340 SNPs
2. Meredith vs. Louis:
   • Chr 2 4.2 cM 909 SNPs
   • Chr 10 5.3 cM 451 SNPs
   • Chr 12 4.0 cM 474 SNPs
   • Chr 15 5.0 cM 340 SNPs
   • Chr 17 4.9 cM 304 SNPs

Answer 1

Although we can say on average two cousins of some distance will share x% of their DNA, the reality is that DNA is passed down through each generation in a chunky fashion. This blog post gives a pretty good explanation. Past 4 or 5 generations back, there is an increasingly high probability that you will not have any detectable DNA from a particular ancestor. As cousins become more distant, the chance that both cousins have detectable DNA from a particular ancestor becomes more and more unlikely. Of course, you have many more distant cousins, so you still will match many distant cousins, but you won't match many more. Not only do the two cousins both have to have detectable DNA from the common ancestor(s) they have to have the same or overlaping segments of DNA from those ancestors. Even if they both have detectable DNA from the common ancestors, the DNA they have may be on different chromosomes, or different areas of the same chromosome.

So the quick answer to your question is that 5th cousins once removed are likely to not have any detectable DNA in common.

Another thing to be careful of is minimum segment size. Because of the way the matching algorithms work with unphased DNA data, it is quite possible for two people to appear to share a common small segment, but in fact they don't. It's only an artifact of the matching algorithms on unphased data. For those who are unfamiliar with the term "phased", this web page may help.

I did some experiments with DNA of closely related individuals, where I phased, or at least partially phased their data. When the phased data was then matched (which is much more reliable), many of the small matching segments turned out to be false segments. Segments 5cm and under were not very reliable. Segments 5 to 7cm were more reliable. Segments 7cm to 10cm were pretty reliable, and segments 10cm and more were extremely reliable.

Answer 2

5th cousins once removed, share an average of 0.024% of their DNA.

Autosomal DNA tests can be used to confirm relationships with a high level of accuracy for parent/child relationships and all relationships up to the second cousin level. For relationships other than parent/child relationships additional contextual and genealogical information is required to confirm the nature of the relationship.

For genealogical relationships between second cousins once removed and 5th cousins a more careful approach is necessary and data needs to be collected from multiple family members. For relationships at the 4th cousin once removed to 5th cousin level you may need to test 10 to 20 or more first and second cousins and see how much autosomal DNA they share with a potential 4th cousin once removed or a potential 5th cousin in order to have sufficient data to generate a statistically significant average amount of autosomal DNA that is shared among the entire group, assuming that you are dealing with a non-endogamous population. See Tim Janzen's summary at http://blog.23andme.com/ancestry/who-were-the-parents-of-jacob-youngman for an example of this type of quantitative approach. For endogamous populations, genealogical relationships are frequently difficult to estimate beyond about the 2nd cousin level of relationship and require careful analysis.

Genealogical relationships beyond the 5th cousin level of relationship are more difficult to prove with autosomal DNA testing and, as a general rule, these can only be approached using triangulation. In some cases Y-DNA and mtDNA data may also be of help.

http://isogg.org/wiki/Autosomal_DNA#Accuracy_of_tests

http://isogg.org/wiki/Autosomal_DNA_statistics#Table

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