Interpreting Your DNA Results
A DNA result is only as useful as your interpretation of it. The key is to treat each match as evidence to be tested against documents, not as a finished answer.
Autosomal matches and centimorgans
Autosomal matches are usually measured in centimorgans (cM). The National Human Genome Research Institute defines a centimorgan as a unit of genetic linkage: one centimorgan corresponds to a one per cent chance that two markers on a chromosome will be separated by crossing over in a single generation. In practice, the more centimorgans you share with someone, the closer the relationship is likely to be.
Because DNA is inherited somewhat randomly, the same relationship can produce quite different amounts of shared DNA. The Shared cM Project, a crowdsourced study led by genetic genealogist Blaine Bettinger, collects real examples to show the range for each relationship, and tools such as DNA Painter’s Shared cM Tool let you enter a number and see which relationships fit.
Y-DNA matches
For Y-DNA, results are compared marker by marker. Two men who match on all or almost all of their STR markers probably share a paternal ancestor within a genealogical timeframe; a growing number of differences suggests a more distant connection. SNP testing then places each line on a branch of the Y-DNA tree, which can confirm or rule out a link.
Haplogroups
A haplogroup is a group of people who share a common ancestor on the paternal or maternal line, identified by shared SNP mutations. Haplogroups are excellent for showing whether two lines could be related at all, but most are thousands of years old, so on their own they rarely answer genealogical questions.
Common mistakes
- Treating an ethnicity estimate as proof of a particular ancestor.
- Assuming the relationship from a cM figure alone without checking the range.
- Ignoring the possibility of more than one shared ancestor, especially in close-knit communities.
- Copying other people’s family trees without checking the sources.
Ready to compare many testers? See how to organise a DNA study.