DNA testing has transformed British family history research. A simple swab can reveal unknown cousins and break down brick walls, but a list of matches with cryptic centimorgan figures can overwhelm. The key is to interpret the data methodically, triangulate matches, and contact relatives with care.

What your shared centimorgans actually tell you

Centimorgans (cM) measure shared DNA. As a rough guide:

• Parent and child: 3,400–3,600 cM
• Full siblings: 2,400–2,900 cM
• Grandparent and grandchild: 1,150–1,750 cM
• First cousins: 550–1,150 cM
• Second cousins: 200–550 cM
• Third cousins: 90–200 cM
• Fourth cousins: 35–90 cM

But ranges overlap. A 200 cM match could be a second cousin or a half first cousin once removed. Treat predicted relationships as a starting point.

In Britain, large databases mean thousands of distant matches. Many share 10–20 cM, pointing to a common ancestor born in the 1700s. DNA inheritance is random: you might inherit no measurable DNA from a great-great-grandparent, so some known cousins won't appear. For instance, if you share 350 cM, that could be a second cousin or a first cousin twice removed. Look at the number of shared segments as well as total cM: closer relatives share more, larger segments.

Building a picture with triangulation

Triangulation confirms a DNA connection. It involves three or more matches who share the same segment on the same chromosome and also share a common ancestor. Because a segment is passed down intact, if you and two cousins overlap on a segment, it almost certainly comes from that ancestor.

Use a chromosome browser or shared match lists. Start with close matches: note which ones share DNA with each other. If three matches on chromosome 7 between 20 and 40 million all have trees pointing to a Devon couple, you have a triangulated group. Some testing companies offer automatic clustering tools that group matches for you, but manual triangulation is more reliable. Keep a spreadsheet of chromosomes, positions, and cM amounts. This turns a confusing list into a structured map.

Making sense of unexpected matches

Not every match fits your tree. You might find a close match with no obvious connection, suggesting an adoption, illegitimacy, or name change. Don't panic. Check the cM amount and predicted relationship, then look at shared matches to assign the match to your maternal or paternal side. Sometimes it's just a missing record. If you have a match that doesn't fit, consider whether a grandparent or great-grandparent had a second family, or whether a surname changed.

Be aware of endogamy, where communities intermarry over generations, causing extra shared DNA. Some British rural parishes or religious groups show this. A half-relationship can also look like a more distant full relationship. If stumped, test an older relative to phase your DNA and separate sides.

Reaching out to DNA relatives

Contacting a match can be nerve-wracking, especially if you suspect a secret. Be polite, concise, and respectful. Introduce yourself, mention the shared cM and predicted relationship, and explain what you seek. Offer to share your tree first. Ask if they are open to discussing family history, with no pressure to reply. If you receive a reply, take it slowly. Build trust before asking for sensitive information.

Use the platform's messaging system, not social media, unless permitted. Be prepared for silence: some test for health reasons only. If writing about adoption or an unknown parent, be gentle. Avoid assumptions. Keep a record of contacts. A warm, patient approach works best.

Using DNA alongside traditional records

DNA is a guide, not proof. Always corroborate with documentary evidence. For British research: civil registration from 1837, census returns 1841–1921, parish registers, wills, and electoral rolls. DNA might point to a county or surname, but you still need the paper trail.

For example, a match might suggest a connection to a Norfolk family. Search the 1851 census for that family and look for a missing ancestor. Or a match could confirm a great-grandfather, but you should still find a baptism or marriage certificate. Use DNA to test hypotheses, not replace them. Combined with archival research, it can break down brick walls.