When DNA Finds a Cousin but the Trees Guess the Parents

Using Ancestry ThruLines wisely without letting it rewrite my Peterson family history

Why This Post?

I want to share a small but important lesson from my own Peterson research. DNA can be a powerful tool for breaking brick walls, but it can also create false confidence when DNA evidence gets blended with unproven online trees.^1,5

Suggested Family Connections

What ThruLines Is Actually Showing

Ancestry’s ThruLines feature works by combining two things: your DNA matches and the family trees connected to those matches. When Ancestry sees that you share DNA with people who descend from a particular individual, and when enough public trees connect that individual to someone in your own tree, ThruLines suggests a relationship. What it does not do is evaluate whether the parent-child links inside those trees are actually supported by records. In practice, this means ThruLines reflects consensus, not proof.²

My Peterson Example With One Proven Link and One Unproven One

In my own research, Robert Peterson is a documented son of John Peterson and Ruth Pyle. That relationship is supported by records and serves as a solid anchor in my tree. The difficulty arises when ThruLines suggests that Tobias Peterson is also the son of John and Ruth. The DNA matches connected to Tobias are real, but the sources attached to Tobias in public trees do not name John and Ruth as his parents. Instead, the suggested relationship relies on Tobias’s approximate birth year and geographic proximity to John and Ruth. Plausible, yes. Proven, no.

Why the DNA Can Be Right Even If the Trees Are Wrong

At the distance involved here, small amounts of shared DNA are exactly what we expect. Matches in the range of roughly 10 to 25 centimorgans often represent distant cousins who share a common ancestor several generations back. At that level, DNA can reliably tell us that two people are related, but it usually cannot tell us exactly how. A shared DNA segment supports shared ancestry somewhere on the Peterson line, but it does not, by itself, establish a specific parent-child relationship.^3,4

Where Tree-Based Assumptions Take Over

Many of the trees that attach Tobias to John and Ruth rely on census records, indexes, or burial databases. These sources confirm Tobias as a real person in a real place at a plausible time, but they do not identify his parents. Once one tree makes the leap to assign Tobias as a son, that assumption gets copied into other trees. ThruLines then treats repetition as reinforcement, even though the underlying claim remains undocumented.

The Key Takeaway

Ancestry is showing what many people believe, not what has been proven. ThruLines can be a helpful tool for identifying DNA clusters and possible family connections, but it is not a proof engine. The discipline to pause, evaluate the sources, and refuse to elevate a guess into a fact is at the heart of careful genealogical work. Even when Ancestry’s interface makes it feel otherwise, the evidence still needs to stand on its own..^1,5

Reframing the Tobias Question

Instead of asking whether Tobias is the son of John and Ruth, a better question is how Tobias fits into the broader Peterson family network. Tobias could be a nephew, a cousin, a grandson through an undocumented child, or part of a related Peterson line that shares an earlier, unidentified ancestor. The DNA supports shared ancestry, but it does not force a single explanation. Leaving Tobias “related but unplaced” is not a weakness. It is responsible research.

How I’m Using ThruLines Without Letting It Rewrite My Tree

I’m not rejecting ThruLines. I’m using it for what it does well: identifying clusters of related people and pointing me toward the right surname and community. At the same time, I keep documented relationships, like Robert Peterson’s connection to John and Ruth, clearly separated from unproven ones. DNA guides my research. Records determine what earns a place in my tree.

Why this matters

DNA is not the problem. Assumptions are. When we let DNA point us toward the right families and places, then slow down and demand documentation before assigning parentage, we end up with conclusions that can stand up over time. ThruLines can be a helpful road sign, but it is not the destination.

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Footnotes

1. Ancestry Support, “AncestryDNA ThruLines,” Ancestry.com.
2. Ancestry Support, “How ThruLines Works,” Ancestry.com.
3. Ancestry Support, “Understanding AncestryDNA Match Categories,” Ancestry.com.
4. DNA Painter, “Shared cM Tool,” https://dnapainter.com/tools/sharedcm.
5. Roberta Estes, “Ancestry’s ThruLines Dissected: How to Use ThruLines and Not Get Bit by the Gators,” DNAeXplained – Genetic Genealogy, March 11, 2019.
   https://dna-explained.com/2019/03/11/ancestrys-thrulines-dissected-how-to-use-and-not-get-bit-by-the-gators/