DNA Testing for Genealogy: A Practical Guide

By Mark Kennedy

July 2025

 

1. Why DNA Testing Matters for Genealogists

Over the past decade, DNA testing has become one of the most powerful tools in a genealogist’s toolkit. It can:

• Break through brick walls when records are missing or inconsistent
• Confirm or challenge family relationships
• Uncover new cousins or entire branches of your family tree

DNA testing doesn’t replace traditional research – it complements it. Think of it as a torch lighting up areas of your ancestry that documents alone can’t reach.

Since the early 2000s, more than 40 million people worldwide have taken direct-to-consumer DNA tests. Some do it to build their family tree, others to explore health risks, others out of curiosity about their ancestry – and many receive surprising results.

This guide explains:

1. Types of DNA tests
2. Choosing a testing company
3. Interpreting your results
4. Sorting through DNA matches

 

2. Understanding the Types of DNA Tests

There are three main types of DNA tests used in genealogy, each serving different purposes:

 

2.1 Autosomal DNA (atDNA)

• Most common test, used by Ancestry, 23andMe, MyHeritage, and others
• Analyzes DNA from both parents – useful for tracing ancestry 5–7 generations back
• Best for identifying cousins and building family trees
• This is the focus of this guide

 

 

2.2 Y-DNA

• Tracks paternal line (father’s father’s father…)
• Available to biological males only
• Useful for surname studies and deep ancestry
• Not as helpful for recent genealogy
• Offered by FTDNA

 

2.3 Mitochondrial DNA (mtDNA)

• Tracks maternal line (mother’s mother’s mother…)
• Inherited by all children, but only passed on by females
• Changes very slowly – best for ancient origins, not recent cousins
• Also offered by FTDNA

 

Summary: Y-DNA and mtDNA are useful for specific research questions. Autosomal DNA is the most practical test for most people.

 

 

 

3. Choosing a DNA Testing Company

 

Company	Strengths
AncestryDNA	Largest database, excellent tree integration
23andMe	Strong ethnicity reports, health information
MyHeritage	Strong European database, AutoCluster tool
FTDNA	Long-term sample storage, Y-DNA & mtDNA
Living DNA	British Isles sub-regional breakdown
GEDmatch	Free tool that accepts uploads from all sites

 

Important:

• AncestryDNA and 23andMe don’t allow uploads from other companies
• MyHeritage, FTDNA, and GEDmatch do allow uploads
• Test first with Ancestry or 23andMe if you want the option to upload elsewhere
• Look out for sales (Black Friday, St. Patrick’s Day, Father’s Day, etc.)

 

 

4. Interpreting Your DNA Results

Once your results are back, your DNA match list will be prioritized by the amount of shared DNA and will look something like this:

Each match includes:

• Username or display name
• Estimated relationship (e.g. 2nd–3rd cousin)
• Shared DNA in centimorgans (cM)
• Family tree availability
• List of shared matches,

 

Likelihood of Shared DNA

4.1 Centimorgans (cM): What They Mean

Relationship	Approx. Shared cM
Parent–Child	~3,400 cM
Full Siblings	2,300–3,000 cM
1st cousins	~800 cM
2nd cousins	200–400 cM
3rd cousins	75–150 cM
4th cousins	20-90 cM
Below 10 cM	Distant or unreliable

 

DNA inheritance is random – shared cM values vary even between same-level cousins. Matches under 7 cM are often false positives.

 

 

 

5. Sorting Through DNA Matches: Step-by-Step

5.1 Start With What You Know

Upload what you know about your own tree first

• Living people remain hidden by default
• Helps you get more useful suggestions from Ancestry.com

Begin by identifying any matches you already recognize:

• First cousins, second cousins, known relatives on specific lines.
• Label or “star” these matches and note how you’re related.
• This creates anchor points – confirmed relationships to help orient others.

Why it matters: These known matches are essential for triangulating unknown matches later.

 

5.2 Use Shared Matches

Click on any DNA match to view others who also match that person. This allows you to form genetic clusters – groups of people who likely descend from a common ancestor.

• Shared matches suggest a common ancestor
• Begin building genetic clusters around these connections
• For example:
  • If your known cousin on your maternal grandfather’s line shares DNA with five other people, those five are likely also related through that same lineage.
  • Group and label them accordingly.

 

5.3 Build Genetic Clusters

Sort your matches into rough “genetic groups” using:

• Shared match overlaps
• Family tree clues (names, places, dates)
• Surnames or locations in profiles

You can keep track of this using:

• Ancestry’s coloured dot system (assign colours to maternal, paternal, and sub-branches)
• A spreadsheet (cM values, tree notes, match names, cluster codes)

Goal: Organize your matches into clusters that represent each of your grandparents’ lines(e.g., maternal grandfather’s line) – or further back.

5.4 Prioritize Effectively

Focus on matches that:

• Share more than 100 cM
• Have public family trees
• Appear active or responsive

Avoid deep dives into matches with tiny cM values (less than 10 – 15) or no trees – unless they’re part of a key cluster.

 

5.5 Work From the Top Down

Start with matches that share more than 50 cM. These are typically 2nd to 3rd cousins and more likely to be genealogically useful.

• Examine their trees: Look for familiar names, places, or ancestral surnames.
• Use the “ThruLines” tool if available: Ancestry attempts to place matches into your tree using shared ancestors.

Be cautious with distant matches (below 20 cM), as these may be false positives or too far back to be traceable.

 

5.6. Evaluate the Trees

Many matches don’t have trees—or have private or limited trees. For those that do:

• Check for overlapping names or locations.
• Look for common ancestors who may appear in multiple trees.
• Note inconsistencies or gaps.

For matches without a tree:

• Review their ethnicity estimates, locations, or surnames in their usernames.
• Consider messaging them to request access or more information.

 

 

5.7 Contacting DNA matches

• Ancestry has an encrypted messaging app.
  • You can set your DNA results and profile to private or public, controlling whether strangers can see or contact you via DNA matches
  • You decide who can message you: adjust settings to only allow matches, contacts, or block specific users
  • Only you and the intended recipient can read your messages; Ancestry itself cannot decrypt them
• Tips
  • Start with shared interest:
    • Mention your shared DNA match and why you’re reaching out
  • Be specific:
    • Reference the amount of DNA you share and any surname, place, or ancestor you’re investigating
  • Keep it brief:
    • A short, clear message gets more responses than a long one
  • Be polite and non-intrusive:
    • Assume they’re new to this and might not be actively researching
  • Offer your tree:
    • Mention if you have a tree online and are happy to compare
  • Ask a simple question: e.g., “Do you have any Callaghans from Limerick in your family?”
  • Be patient:
    • Many matches don’t check messages regularly—follow up after a few weeks
  • Respect privacy:
    • Not everyone is ready to share details or respond

 

 

 

6. Making the Most of DNA Platform Tools

Although Ancestry doesn’t show segment data, tools can still help map and model relationships. For more advanced sorting and clustering, you might use tools like:

 

6.1 Ancestry Features

• Shared Matches: Helps you cluster
• Notes & Color Coding: Track match lines visually
• ThruLines: Suggests possible connections based on shared ancestors in trees

 

6.2 My Heritage Features

• Theory of Family Relativity: Suggests how you and a match may be related
• Auto Cluster Tool: Automatically sorts matches into genetic clusters

 

6.3 GEDmatch & DNA Painter (Third Party Tools)

• GEDmatch:
  • Advanced tools and matching from multiple platforms
  • https://www.gedmatch.com/

 

• DNA Painter:
  • Maps segments, helps estimate relationships
  • https://dnapainter.com/

 

• Leeds Method:
  • Manual clustering technique for 2nd–4th cousin matches
  • https://www.danaleeds.com/the-leeds-method/

 

 

7. Tips for Reliable DNA Research

 

7.1 Look for Patterns, Not Guarantees

DNA doesn’t give you answers—it gives you clues. Use the following principles:

• Probability over possibility: Focus on what’s most likely, not just plausible.
• Use evidence over hunches: Use trees and records to back up DNA connections

• Use triangulation: More than one person matching you and each other in a cluster makes the connection stronger.

• Don’t rely solely on one match – look for clusters of evidence

 

7.2 Document Your Work

Keep a DNA match log or spreadsheet:

• Match name and cM, predicted relationship
• Hypothesized common ancestor
• Cluster or group id.
• Notes on tree content or correspondence
• This helps you track progress, revisit leads and share findings with relatives.

8. Be Ready for Surprises

 

DNA results can reveal:

• Unknown half-siblings
• Adoptions
• Misattributed parentage (NPE – non paternal events)

Handle these unexpected findings with sensitivity and discretion.

 

9. Key Takeaways and Strategy

Sorting through DNA matches takes time, patience, and methodical effort. It’s detective work, often requiring both genetic and traditional genealogy. By building out clusters, confirming shared ancestors, and comparing trees, you gradually turn a mess of matches into meaningful family connections. DNA doesn’t replace the paper trail—it enhances it. When used well, it can break down brick walls and bring the past to life in ways that documents alone never could.

• Prioritize testing of older living relatives – they hold more of your shared DNA
• Test with AncestryDNA first, then upload to others for broader reach
• Focus on closest matches with trees and shared matches