DNA Testing, The Difference it Makes Testing One Generation Back: Case Study

It is often recommended to give a DNA test to the oldest generation possible.  But how much does this really matter?

When I was starting out in DNA research a few years ago, I thought that testing a child versus their parent didn’t really matter.  I thought because a child had half of each parent’s DNA, the child would have essentially the same DNA matches as each of his parents, just not the really distant ones.  And each match length would be exactly halved.  In actuality, this is not the case.  A child does indeed inherit exactly half of their DNA from each parent, but not necessarily 25% from each grandparent, and not necessarily 12.5% from each great-grandparent, and so on back through the generations.  It varies.  So some matches on a parent’s DNA match list will appear on a child’s list, but some will not.  But how many will not?

During the Black Friday AncestryDNA sale last year, I ordered DNA testing kits for two relatives, a mother and her daughter.  The results just came back.  I have done a good deal of research on this family, so have pretty complete trees for both that I have linked to their DNA results.  By looking at their shared ancestor hints and overall match statistics, we can get a feel for what a difference a generation can make.

Let us start with Shared Ancestor Hints, a notification that two people who share DNA also share one or more common individuals in their actual family trees.  These are significant, because they can help confirm research and help place other DNA matches in your tree when a match with one of these people is shared in common.  In this case the mother has 38 Shared Ancestor Hints, one of which is her daughter, so let’s consider it 37 individuals.  The daughter has 28 Shared Ancestor Hints, one again is the mother, so let’s consider it 27 individuals.

  • Of the mother’s 37 matches, only 10 are shared with her daughter.  That’s only 27% of her mother’s match hints!
  • An additional two ancestor hint matches on the daughter’s list are supposed matches on her mother’s side, but they are not matches with the mother.  It could be each match is partially identical by descent and partially identical by state (random chance).  But we should be wary of them.  Both are smaller matches anyway, but without the mother’s test, she wouldn’t know to place even less confidence in these matches.

This 27% number considers all the shared ancestor hints.  How about closer matches versus more distant matches?

  • The mother had seven Ancestor Hint matches at “4th cousin” (> 20 cM) level or closer.  These are the ones (as of right now) you can see on the “in common with” list on other matches.  They help determine the side of the family others connect on.
  • The daughter shared only five of these seven matches.

We can look at the mother’s seven largest matches with Ancestor Hints:

  1. Shares 1821 cM with mother, 819 cM with daughter
  2. Shares 215 cM with mother, 99 cM with daughter
  3. Shares 53 cM with mother, 35 cM with daughter
  4. Shares 37 cM with mother, 26.2 cM with daughter
  5. Shares 21.8 cM with mother, NOT a match with daughter
  6. Shares 20.8 cM with mother, NOT a match with daughter
  7. Shares 20.3 cM with mother, 22.3 with daughter (some identical by state here)

As would be expected, the larger matches, 37 cM and greater, with the mother also match the daughter, but some matches in the 20’s do not match the daughter.  Also, looking at the match lengths, the larger matches roughly get halved each generation, but it is certainly not an exact 50% split.

The largest matches do indeed generally get passed down, but we start losing matches pretty quickly.  And just because the smaller matches are smaller does not mean they are not useful.  Those smaller matches in the mother’s list, most of which weren’t passed to her daughter, cover many sides of the family tree and are still valuable in the chance to connect with relatives and learn more about certain branches of the tree.  Significant information is lost between the generations.

So far we have looked at Shared Ancestor Hints.  How about the general match list, not just with hints?

The closest ten common overall matches with the mother:

  1. Shares 1912 cM with mother, 935 cM with daughter
  2. Shares 1821 cM with mother, 819 cM with daughter
  3. Shares 385 cM with mother, 167 cM with daughter
  4. Shares 215 cM with mother, 99 cM with daughter
  5. Shares 132 cM with mother, 49 cM with daughter
  6. Shares 127 cM with mother, 46 cM with daughter
  7. Shares 113 cM with mother, 44 cM with daughter
  8. Shares 113 cM with mother, 53 cM with daughter
  9. Shares 106 cM with mother, 47 cM with daughter
  10. Shares 95 cM with mother, NOT a match with daughter

The first match on the mother’s list that the daughter misses completely is a “Third Cousin”-level match at 95 cM.  This is a significant match, yet the person is completely missing from the daughter’s match list.

Analyzing the next four sets of ten matches with the mother:

  • Matches 11 to 20, 90 to 54 cM, daughter matches 80%
  • Matches 21 to 30, 53 to 49 cM, daughter matches 50%
  • Matches 31 to 40, 49 to 46 cM, daughter matches 40%
  • Matches 41 to 50, 46 to 44 cM, daughter matches 30%

The common match rates have gotten pretty poor by the 50th match at 44 cM.  There is still a lot of good information that could be gained from a match in this range, but the matches are significantly dropping out already.

So for this particular case of a mother and her daughter, the daughter has only 27% of her mother’s Shared Ancestor Hints, she shows no match to an individual that shares 95 cM with her mother, and by 44 cM the daughter retains only about 30% of her mother’s matches.  These all demonstrate that a great deal of information is lost from one generation to the next.

Of course, there is a lot of randomness here, and not all cases are the same.  The randomness of inheritance, the sizes of families at every generation, how likely it is someone in your distant family would take a DNA test or be at all interested in genealogy, all factor in here.  But please consider asking older-generation family members to take DNA tests before it is too late.  They carry a wealth of information about your family, and not all of it may be passed down to later generations.

Posted in DNA

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