Second cousins usually share about 3.1 percent of their dna, though real dna matches can fall a bit below or above that average.
Why This Second Cousin Dna Question Matters
If you are staring at a DNA test result that labels someone as a second cousin, you are mainly asking one thing: how closely related are we in genetic terms. The answer guides family tree work and sets expectations about shared ancestors.
Genetic genealogy companies turn raw genetic markers into a simple number. That number is either a percentage of shared dna or a total amount of shared centimorgans, usually written as cM. Centimorgans measure how much of your chromosomes match another person.
Family Relationships And Typical Shared Dna
Before zooming in on second cousins, it helps to see where they sit on the wider family ladder. The chart below shows the expected percent of shared autosomal dna for several common relationships, plus typical centimorgan ranges quoted by major testing providers.
| Relationship | Average Shared Dna % | Typical Shared cM Range |
|---|---|---|
| Parent / child | 50% | About 3,300–3,900 cM |
| Full siblings | 50% | About 2,200–3,100 cM |
| Grandparent / grandchild | 25% | About 1,300–2,300 cM |
| First cousins | 12.5% | About 575–1,330 cM |
| Second cousins | 3.125% | Roughly 75–360 cM |
| Third cousins | 0.78% | Roughly 30–215 cM |
| Fourth cousins | 0.20% | Roughly 0–90 cM |
The average percentages in that table come from classic population genetics work and are summarised neatly on the consanguinity page of a well known online encyclopedia. The centimorgan ranges match figures in shared dna charts published by major testing companies such as this 23andMe help page, which shows both expected averages and observed ranges.
How Much Dna Do Second Cousins Share? By Percent And Cms
On paper, second cousins are expected to share 3.125 percent of their autosomal dna. That figure comes from a simple rule: each step between generations cuts the expected shared dna in half. First cousins share grandparents. Second cousins share great grandparents, so the coefficient of relationship works out at one thirty second, or just over three percent.
In practice, dna inheritance is random at the level of segments. So two second cousins can share a bit less or a bit more than that neat number. Genetic genealogy projects and company data suggest that the real shared amount for second cousins usually falls somewhere between about 2.8 and 5 percent of their autosomal dna, which roughly matches a total of 75 to 360 cM of shared segments.
How Much Dna Second Cousins Share On Average
Different sources report slightly different average centimorgan values for this relationship. Some public charts quote an average near 229 cM, others closer to 212 cM, based on large datasets of known second cousin pairs. The central idea is that most second cousin pairs sit in the low hundreds of centimorgans. So if your match shares around 200 to 250 cM with you, second cousin is a strong fit.
That range can overlap with other cousin types, such as first cousin once removed or third cousin in special cases. Testing companies run statistical models on the exact pattern of your shared segments to pick the most likely label. That is why your match list might mark the same person as a close relative, second cousin, or second to third cousin range rather than a single fixed slot.
What Makes Second Cousin Dna Vary So Much
The clean textbook number for how much dna second cousins share assumes perfect averaging over many families. Real families never look that tidy. The amount shared depends on how chromosomes were shuffled during meiosis in each generation from your shared great grandparents down to you and your cousin.
When eggs and sperm form, each parent passes on a mix of their own two chromosome sets. Large blocks move together, and the exact crossover points change each time. That process means siblings do not share the same segments, and it also means that distant cousins may pick up more or fewer matching blocks of dna from the same ancestors.
In one family, two second cousins might share close to 350 cM. In another family, a pair with the same paper relationship might share nearer 80 cM. Both outcomes fit real data. This spread is why genetic genealogists talk in ranges and probabilities, not in single fixed numbers, when they answer a question like how much dna do second cousins share.
Reading Your Dna Test: Percentages Versus Centimorgans
Most consumer tests show two numbers for each match. One is the percent of shared dna, often written as a small figure like 3 percent, and the other is the total shared cM. Centimorgans are a unit that reflects how likely a segment is to be broken up by recombination in each generation. One cM roughly matches one million base pairs in humans, though the exact physical distance varies across the genome.
For second cousins, your report might say something like 3 percent shared dna and 220 cM over ten segments. The more segments you share, and the longer those segments are, the closer the relationship is likely to be in real life. Short scattered segments can show up with much more distant cousins and with people who share older common ancestors.
Match Categories For Second Cousins
Major testing companies group second cousins with nearby relationships because the ranges overlap. A match who shares around 200 cM might be a second cousin, a first cousin once removed, or even a great great aunt in certain pedigree layouts. The label on your match list is a starting point, not a final verdict.
To work out whether a match is truly a second cousin, combine the shared dna number with ages, family locations, and any family tree information. If both of you can name a pair of great grandparents that match, you have confirmed the second cousin connection. If the shared ancestors sit one generation higher or lower for one of you, the relationship shifts to second cousin once removed instead.
Second Cousin Dna Scenarios In Practice
It helps to see how varied second cousin dna can look on a test report. The table below lists some sample shared dna amounts and shows how test providers might interpret them. Numbers here are rounded and simplified, but they give a sense of how much dna second cousins share compared with nearby relatives.
| Shared cM | Approx Shared Dna % | Likely Relationship Zone |
|---|---|---|
| 80 cM | About 1.1% | Distant second cousin or third cousin |
| 150 cM | About 2.1% | Lower end second cousin, third cousin, or second cousin once removed |
| 220 cM | About 3.1% | Typical second cousin range |
| 300 cM | About 4.3% | High sharing second cousin or first cousin once removed |
| 360 cM | About 5.0% | Very close second cousin or other close relative |
| 50 cM | About 0.7% | Likely third cousin or more distant |
Every testing company sets its own match categories and probability bands. A given amount of shared dna always sits inside a cloud of possible relationships, which is why many reports list several options. When the label includes second cousin, the shared centimorgan number will usually land somewhere inside the band shown in this table.
What Shared Dna With A Second Cousin Means For Family History
From a family tree perspective, a second cousin is someone with whom you share great grandparents. Your parent and your cousin’s parent are first cousins. That means the two of you are about the same distance from the ancestral couple, and the dna you share is a collection of segments passed down from that pair and from their own ancestors.
Because the shared dna fraction is small, second cousins can be especially helpful for confirming lines, but they rarely reveal private details about you. A second cousin match can lead you to new surnames, new branches, and fresh records for your great grandparents’ siblings. Many people use second cousins as anchors when mapping clusters of matches in a genetic genealogy project.
Second Cousins, Shared Dna, And Health Concerns
People sometimes worry when they see that second cousins share a measurable amount of dna. In clinical genetics, relationships between second cousins or more distant relatives sit at the low end of consanguinity. The average shared dna of just over three percent means the genetic risk for their children is usually only slightly higher than the background population risk.
Even so, any couple with questions about inherited conditions should talk with a medical doctor or genetic counselor, especially if there is a known condition in the wider family. Screening programmes, carrier tests, and simple pedigree charts can all help clarify risk and options.
Using Second Cousin Dna Matches Effectively
When you know how much dna do second cousins share in theory and in practice, match lists start to make more sense. You can sort your matches by shared cM, mark likely second cousin matches, and then look for shared surnames or locations. As you confirm links, you give testing company algorithms more context for future relatives as well.
If a new test result drops a match in the same centimorgan band as several known second cousins, that person probably connects on the same branch of your tree. Combine that hint with record research, and you may be able to name the shared great grandparents who sit behind the match overall.