Understanding DNA Matches for Genealogy Research

How DNA testing works for genealogy

Consumer DNA testing companies analyse your genetic material and compare it against databases of other testers. When two people share segments of DNA, it indicates a common ancestor.

There are three types of genealogical DNA tests:

• Autosomal DNA: Tests all ancestral lines, effective for about 5-7 generations. This is the most common and useful test.
• Y-DNA: Traces the direct paternal line (father's father's father…). Only available for males.
• Mitochondrial DNA (mtDNA): Traces the direct maternal line (mother's mother's mother…). Available for everyone but changes very slowly.

What are centiMorgans (cM)?

CentiMorgans measure the amount of DNA shared between two people. The more cM you share, the closer your relationship:

• Parent/child: ~3,400 cM (50% of DNA)
• Full sibling: ~2,550 cM (variable)
• Grandparent/uncle: ~1,700 cM (25%)
• First cousin: ~850 cM (12.5%)
• Second cousin: ~210 cM (3.1%)
• Third cousin: ~50 cM (0.8%)
• Fourth cousin: ~12 cM (often undetectable)

These are averages - actual amounts vary significantly. Two people can be related in multiple ways, which increases the shared cM.

Interpreting relationship predictions

DNA testing companies predict relationships, but these predictions are estimates:

• A match of 1,700 cM could be a grandparent, aunt/uncle, half-sibling, or double first cousin
• Shared DNA decreases by roughly half each generation, but with high variability
• Close matches (>400 cM) are almost always correctly identified within a few categories
• Distant matches (<50 cM) could represent many different relationships

Use the published Shared cM Project data to see the probability distribution for any shared cM value. GEDminer's DNA Planner shows expected shared DNA for known relationships in your tree.

Using DNA to break through brick walls

DNA matches can solve problems that paper records cannot:

• Unknown parents: Cluster your DNA matches into groups to identify which side of the family they belong to, then narrow down candidates
• Confirm or disprove hypotheses: If you suspect two families are connected, shared DNA proves it genetically
• Identify adoption or NPE events: Non-paternity events (where the biological father differs from the documented one) often appear as unexpected DNA match patterns
• Reach beyond records: In areas where records were destroyed (wars, fires), DNA may be the only way to connect families

Which relatives should you test?

Strategic testing amplifies your results:

• Oldest generation first: They carry DNA from further back. A grandparent's test effectively tests their parents and grandparents.
• Multiple siblings: Each sibling inherits different segments, so testing all siblings covers more of your parents' DNA
• Distant cousins: A second cousin on your mother's side helps separate maternal from paternal matches
• Both sides: Test relatives from both paternal and maternal lines

GEDminer's DNA Testing Planner analyses your tree to recommend which living relatives to test first for maximum genealogical value.

DNA testing best practices

Make the most of your DNA results:

• Test on multiple platforms: Different DNA databases attract different users. Where supported, transferring your raw data between services expands your match pool.
• Build your tree: DNA matches are most useful when connected to documented family trees
• Respond to messages: Other testers may have the missing pieces of your puzzle
• Stay ethical: Respect the privacy of your matches and their families
• Be patient: New matches appear regularly as more people test

Combine DNA evidence with traditional genealogy research for the most reliable conclusions.