DNA tests for family history: autosomal, Y-DNA and mtDNA explained, plus privacy

TUREFF• •0

Consumer DNA tests have changed family history research, finding relatives that no document could and solving adoption and parentage questions that once seemed impossible. They also raise questions that paper records never did, about privacy and about uncomfortable discoveries. This guide explains the three main types of test, what each can and cannot tell you, and what to think about before you spit in a tube. It does not recommend any particular company.

Autosomal DNA: your relatives on every line

Autosomal DNA is inherited from both parents, roughly half from each, and gets shuffled each generation. It is the most common test sold for family history.

What it shows: a list of matches, other testers in the same company's database who share stretches of DNA with you, with an estimate of how closely related you are.
How far back: very reliable for close relatives such as parents, siblings and first cousins. It becomes less reliable the further back you go; at around the level of fifth cousins, two genuine relatives may or may not share detectable DNA. Most genealogists treat it as useful for roughly the last five or six generations.
Ethnicity or ancestry estimates: these compare your DNA with reference groups and estimate regions of origin. They are best seen as broad, approximate and likely to change as companies update their methods. They are entertaining but less useful for genealogy than the match list.

Y-DNA: the direct paternal line

Y-DNA is carried on the Y chromosome, which passes from father to son almost unchanged.

Only people with a Y chromosome, typically men, can take this test. A woman wanting to explore her father's paternal line can ask a brother, father, paternal uncle or male cousin from that line to test.
It follows one line only: your father's father's father, and so on. That often matches the surname line in cultures where surnames pass from father to children.
It is useful for surname studies and for checking whether two men with the same surname share a paternal ancestor.

Mitochondrial DNA (mtDNA): the direct maternal line

Mitochondrial DNA is passed from a mother to all her children, but only daughters pass it on.

Anyone can take an mtDNA test.
It follows your mother's mother's mother's line.
Because it changes very slowly, it is good for deep ancestry questions but often less useful for finding recent relatives, as matches can share an ancestor many centuries back.

Getting the most from your results

Build your documented family tree alongside your DNA. Matches are clues, and you identify how you are related by comparing trees.
Testing older relatives, such as grandparents or great-aunts, captures DNA one or two generations closer to your ancestors.
Group your matches by which side of the family they appear on.
Contact matches politely, explain what you are researching and accept that many will not reply.

Privacy considerations

DNA is different from other personal data: it cannot be changed, and it reveals information about your relatives as well as you.

Read the terms: check what the company does with your sample and data, whether it is used for research, and how to delete your data and have the sample destroyed.
Law enforcement access: genetic genealogy databases have been used by police to identify suspects, most famously in the 2018 Golden State Killer case in the US. Policies on police access vary between services and have changed over time; check each service's current rules and settings.
Uploads to other sites: some people upload their raw data to third-party sites to find more matches. Each upload means another company's terms and security apply.
Data breaches can happen to any company, so think about what details, such as your full name, you attach to your profile.
Health information: some tests report health-related results. These are not a diagnosis; discuss anything concerning with a doctor or genetic counsellor.
Relatives: your results reveal partial information about your parents, siblings and children, who did not choose to test.

Be ready for surprises

DNA testing sometimes reveals unknown siblings, adoptions, donor conception or that a parent is not who you thought. Before testing, consider how you would feel about such a discovery. If it happens, take your time before contacting anyone, and look for support from people who have been through the same experience or from a counsellor. Have DNA results helped or complicated your research? Share your experience or ask a question in the comments.