If you’ve taken a DNA test with AncestryDNA, MyHeritage, 23andMe or another testing company, you’ll quickly notice that each DNA match has a number beside their name followed by the letters cM. But what exactly are Centimorgans (cMs) and why do they matter for family historians? You might see for example that you share 23 cM with one match, 248 cM with another and 1,765 cM with a close relative. But what do these numbers actually mean?
Understanding centimorgans (often shortened to cMs) is one of the keys to making sense of your DNA results. A centimorgan is a unit used to measure the amount of DNA that two people share. Despite the name, it isn’t a physical measurement like centimetres or miles. Instead, it measures the likelihood that a section of DNA will be inherited together from one generation to the next.
For family historians, you don’t need to understand the science behind genetic recombination. The important thing to remember is simple: The higher the number of shared centimorgans, the more closely you are likely to be related.
Every child inherits approximately 50% of their DNA from each parent, but the exact pieces they inherit are random. This means that while brothers and sisters share the same parents, they won’t inherit exactly the same DNA. As a result, they can share different amounts of DNA with cousins, aunts, uncles and more distant relatives.
That’s why DNA relationships are always expressed as a range rather than an exact number. The following examples show the average amounts of DNA shared between common relatives.
These figures are only averages. Real-life results often vary considerably. Unfortunately, a shared cM number cannot tell you exactly how you’re related to someone. For example, if you share around 220 cM with another person, they could be a second cousin, a first cousin once removed, a half first cousin or another relationship that shares a similar amount of DNA. This is why family trees remain so important. DNA provides the evidence that two people are related, while traditional family history research helps identify exactly how. Many people ignore matches below 20 cM, assuming they aren’t useful.
In fact, smaller matches can be extremely valuable, particularly when researching Welsh families. Because many Welsh communities remained relatively local for generations, it’s common to find several DNA matches who all descend from the same village, chapel or farming family. Individually these matches may only share 15–30 cM, but together they can provide strong evidence that you’ve identified the correct ancestral line. The key is to look for patterns rather than focusing on a single match.
Most DNA testing companies also allow you to view Shared Matches – people who match both you and another DNA match. This can often reveal which side of your family a match belongs to before you’ve even identified the common ancestor. For example, if several matches all descend from families living around Marshfield, St Brides or the Gwent Levels, they may point towards the same ancestral branch.
Combining shared matches with documentary research is often the quickest way to solve family history puzzles. DNA testing has transformed genealogy, but it rarely provides all the answers on its own. Birth, marriage and death certificates, census records, parish registers, probate documents and local history sources all help explain the relationships suggested by DNA.
At Welsh Family History, we regularly combine DNA evidence with traditional genealogical research to identify unknown ancestors, confirm family connections and help clients overcome long-standing brick walls. Looking at hundreds or even thousands of DNA matches can feel overwhelming. Whether you’re trying to identify an unknown ancestor, understand an unexpected DNA match or make sense of your shared centimorgan amounts, we can help.
Our experienced researchers combine DNA analysis with traditional Welsh family history research to uncover the stories behind your results and connect them to your family tree. Get in touch to find out how we can help you discover more about your Welsh ancestry.
