How do DNA testing companies find your distant family?

DNA expert Debbie Kennett explains how DNA testing companies identify your relatives - and how far you can trust the results

Blind Man's Buff

The companies that offer a relative-finding autosomal DNA test are: AncestryDNA23andMeFamilyTreeDNA and MyHeritage DNA. A fifth, Living DNA (also sold by Findmypast), is beta-testing a family network service.

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Each company has its pros and cons, and they all offer a range of features that help you to work with your matches.

The number of matches you get will vary from company to company. Each company has its own proprietary database, and you will find some cousins in one database who have not tested elsewhere.

Although the size of the database determines the number of matches you get to some extent, each company also uses different algorithms and sets different thresholds for what it considers to be a match.

The amount of DNA shared between you and your cousins is measured in units known as centiMorgans (or cM). Current tests are run on microarrays or chips that cover about 700,000 markers scattered across the genome.

However, this represents a fraction of the total variation in our genomes, and there are some regions that have very poor coverage on the chips. This introduces the possibility of errors when inferring matches.

Some of the companies use a technique known as phasing to sort the DNA letters – all those As, Cs, Ts and Gs – onto the maternal and paternal sides.

If phasing is not done then the algorithms will zigzag between letters on the paternal and maternal chromosomes to produce false matches.

The companies that use phasing are able to set a lower threshold for matching, because they feel more confident that the matches are real.

The companies try to get the balance right, so that they are reporting a reasonable number of matches without too many false ones.

While matches with close relatives sharing large amounts of DNA can be called with confidence, many matches with more distant cousins are likely to be false.

I’ve been able to test both of my parents and I’ve found that 26–36 per cent of my matches at the different companies don’t match either of my parents.

It is mostly the distant matches that are affected, particularly when matches share 15 cM or less.

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Some firms assign confidence levels to matches. For instance, at AncestryDNA matches sharing 6–16 cM are rated as “moderate” with only a 15–50 per cent likelihood of sharing a single recent common ancestor, while MyHeritage has “moderate” or “low” confidence in matches under about 30 cM, stating, “This match may be a distant relative, but there is also a small chance that it is not related to you.”