Which Relatives Should Take a DNA Test First?

Why the oldest generation comes first

Autosomal DNA, the kind used for relative matching and ethnicity estimates, comes half from each parent. Each time it passes down a generation it is shuffled and cut in half again. That shuffling is called recombination. On average you carry about 50% of each parent's DNA, about 25% of each grandparent's, and about 12.5% of each great-grandparent's. The real amounts vary because the shuffle is random.

So every generation loses pieces of the family's DNA. The half of your mother's DNA that you didn't inherit isn't gone, but it isn't in you either. It's in her, and some of it is in your siblings. Distant cousins who share only those missing pieces will match your mother and not you.

That is why genealogists say to test the oldest living generation first. A grandparent carries DNA from the great-grandparents that may never have reached you, so they'll match more of the distant cousins who descend from those lines. MyHeritage's education team makes the same point: older relatives share more DNA with your ancestors, which raises the chance of useful matches.

There is also a plain practical reason. Older relatives won't always be able to test later. If someone in your family is in their eighties or nineties, ask them first.

A practical testing order for genealogy

You don't have to test everyone. Start at the top with whoever is living and willing, then fill in the side of the family your research question is about.

How testing both parents splits your matches

If you've ever stared at a list of hundreds of DNA matches, you know the first question is "which side is this person on?" A tested parent answers that quickly. If a match shares DNA with you and also with your mother, the match is almost certainly on your mother's side. If they match you but not your mother, look to your father's side, or to a shared match who is connected some other way.

With both parents tested, most of your matches sort themselves. Your parents will also have matches you don't have at all, which is another reason to test them rather than only yourself.

Some sites now estimate the split without parents. Ancestry, for example, can sort many matches into "Parent 1" and "Parent 2" using its SideView technology even if neither parent has tested. Ancestry's help page says the groups get Maternal and Paternal labels once a parent tests, and that many matches may stay unassigned until enough relatives have tested. A tested parent is still the most direct way to know which side a match is on.

When parents or grandparents can't test

If a parent has died or declines, you can rebuild much of their DNA from people who inherited from them or share their parents.

• Their other children. Each child carries a random half of the parent's DNA. On average, one child carries half, two children together carry about three-quarters, and three carry about seven-eighths. Each extra child adds less than the one before.
• Their siblings. Your aunts and uncles share the same parents, so their matches overlap heavily with the matches your parent would have had.
• Known cousins. A first cousin on your father's side who tests can help separate your father's matches from your mother's.

Siblings can also get surprisingly different ethnicity estimates for the same reason. See why siblings get different ancestry results.

Choosing who to test for Y-DNA and mtDNA

Two other test types follow one straight line each. They work differently from autosomal DNA, so "oldest first" matters less. These lines change very little from one generation to the next.

• Y-DNA passes from father to son, so only males can take a Y-DNA test. It traces the direct paternal line, which in many cultures is also the surname line. If you're a woman researching your father's surname line, ask your father, a brother, a paternal uncle, or a paternal uncle's son. To trace a different surname line, such as your mother's father's, you need a man who descends from that line through sons only, like your mother's brother.
• Mitochondrial DNA (mtDNA) is carried by everyone, but only mothers pass it on. You, your siblings, and your mother all share your mother's mtDNA line. For your father's mother's line, test your father or one of his siblings.

The exception to "less urgent" is when someone is the last living person carrying a line. If your great-uncle is the only man left on a particular paternal line, test him while you can. For what these results do and don't show, see haplogroups explained.

For health questions, the rule is different

Genealogy rewards testing the oldest relatives. When the question is an inherited health risk, the starting point is usually the person who actually has or had the condition.

The National Cancer Institute puts it plainly: "It is generally recommended that, when possible, genetic counseling and testing for a hereditary cancer syndrome begin with a family member who has had cancer." Here is why that order matters:

• Finding the variant first makes everyone else's test clearer. If the affected relative's test finds a disease-causing variant, other relatives can be tested for that specific variant. The CDC calls this cascade testing. It describes it as informing family members of a genetic condition found in the family and offering them testing, starting with the closest relatives such as parents, siblings and children. The CDC names hereditary breast and ovarian cancer, Lynch syndrome and familial hypercholesterolemia as examples.
• A negative result in an unaffected person can mislead. If no variant has been identified in the family, NCI calls a negative result an "uninformative negative." The family may still carry a variant the test didn't find.

If the affected relative has died or doesn't want to test, a genetic counselor can help you decide who else to test and what a result would mean. Note that consumer genealogy tests are not built for this job. NCI notes that a consumer BRCA test looks at only three variants and misses about 80% of cancer-causing BRCA variants. Talk with a clinician or a genetic counselor before acting on any health finding, and don't change medication or screening based on a consumer result alone.

It also helps to bring a written family health history to that appointment. Our guide to collecting your family health history covers what to ask.

Some families decide one member should have a complete genome on file as a reference for later reanalysis. Whole genome sequencing doesn't give you a genealogy match list, and it doesn't replace clinical testing ordered by a clinician.

Every tester decides for themselves

Buying a kit for your grandmother is a kind gesture. The decision to spit in the tube is still hers, and so is the data. Before anyone tests, talk through what it can reveal:

• Unexpected relatives. Matching can surface unknown half-siblings, adoptions, or a different biological parent. MedlinePlus warns that sharing results can reveal relationships that were previously unknown. Make sure an older relative is comfortable with that possibility, and agree in advance how you'll handle a surprise.
• Database settings. Each tester should choose whether to appear in match lists, whether to upload to other sites, and whether to allow law enforcement matching where a site offers that choice. See police and genetic genealogy databases.
• Who manages the account. Many companies expect adults to register their own kit. Ancestry, for example, requires the test taker to register on their own account. They can then name you as a manager. Its exceptions are minors and incapacitated adults whose power of attorney holder manages the account. Check each company's current rules before you order.

Remember too that one person's test reveals something about everyone they're related to. If a relative says no, that's the answer. Our article on how one relative's test affects the family covers this in more depth.

Bottom line

For genealogy, ask the oldest living relatives first. Try for both parents if you can, and use aunts, uncles and siblings to fill gaps. Choose Y-DNA and mtDNA testers by which line you want to follow. For a health question, start with the relative who has the condition and involve a genetic counselor. In every case, the person whose DNA it is gets the final say.

Common questions

Medical disclaimer. GenomeTesting.org publishes educational content about genetics and consumer genomic testing. Nothing here is medical advice, a diagnosis, or a substitute for care from a qualified clinician or genetic counsellor. Decisions about testing, treatment, or interpreting your own results should be made with a healthcare professional who can see your full history.

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