DNA Testing and Traditional Genealogy: How the Two Fit Together

What autosomal, Y-DNA and mitochondrial tests can and cannot tell you, how to use DNA matches with a documented family tree, and where the limits lie.

A DNA test does not build a family tree. It confirms, contradicts and extends the tree you build from records. Used together, the two methods are far stronger than either alone. Here is what each test type offers and how to combine it with paper research.

The three test types

• Autosomal DNA (the standard test from Ancestry, MyHeritage, 23andMe, FamilyTreeDNA) looks at the 22 pairs of non-sex chromosomes you inherited from both parents. It finds relatives on all lines, reliably up to about third or fourth cousins and unreliably beyond that.
• Y-DNA follows the direct paternal line, father to son, because the Y chromosome passes almost unchanged. It is useful for surname studies and for deep ancestry, and only men can take it.
• Mitochondrial DNA follows the direct maternal line, mother to child. It changes slowly, so matches can be very distant and it is rarely useful for recent genealogy.

What a match actually tells you

An autosomal match reports how much DNA you share, in centimorgans (cM). Roughly:

• 1,700–2,600 cM: a parent, child or full sibling.
• 700–1,300 cM: a grandparent, aunt, uncle, half-sibling or first cousin.
• 200–400 cM: a second cousin or equivalent.
• 40–150 cM: a third cousin or equivalent.
• Below 20 cM: possibly a distant cousin, possibly nothing.

The amount of shared DNA suggests a range of relationships, never a single one. Only a documented tree tells you which relationship it is.

Working from matches to the tree

1. Sort matches by shared cM and start with the closest ones you cannot identify.
2. Look at the trees your matches have posted and find surnames and places you recognise.
3. Group matches by shared matches; a cluster usually corresponds to one ancestral couple.
4. Build the match's tree from records back to the point where it meets yours.
5. Record the connection in your tree with the DNA evidence as a source.

The last step is the one people skip. A DNA match is a source like any other: cite the test company, the amount shared and the date.

Where DNA overrules paper

• Misattributed parentage: a father in the record who was not the biological father. DNA finds these; records never do. Expect one somewhere in every tree, and think about how you will handle it before you find it.
• Adoptions and foundlings with no documentary trail.
• Two families with the same name in the same village: matches with one cluster and not the other settle the question.

Where paper overrules DNA

• Any relationship beyond about four generations: too little shared DNA survives.
• The exact relationship: DNA says second cousin or first cousin once removed; a marriage record says which.
• Names, dates and places: DNA has none.

Ethics and privacy

A test can reveal facts about living relatives who did not take it. Before you share results, think about who could be affected. Our article on privacy in family trees applies doubly to DNA.

Putting it on paper

When a DNA connection is confirmed by records, it belongs in the same GEDCOM file as everything else, with the citation on the relationship. The chart and the book that come out of that file then show a family verified two ways.