Yes, third cousins are blood related. You and a third cousin share a set of great-great-grandparents, meaning you descend from the same ancestor through different branches of the family. The amount of shared DNA is small but real: on average, third cousins share about 0.78% of their DNA.
How Third Cousins Are Connected
Third cousins share one set of great-great-grandparents. To visualize this, start with a couple who had children. Those children had children (your grandparents’ generation), who had children (your parents’ generation), who had you. A third cousin followed the same path down a different branch from that same original couple. That puts four generations between you and your common ancestors.
This is a genuine blood relationship, not a relationship by marriage. Every link in the chain from you back to those shared great-great-grandparents is a biological parent-child connection. The “third” simply refers to how many generations removed you are from the common ancestor, not to the strength or legitimacy of the connection.
How Much DNA Third Cousins Actually Share
Third cousins share roughly 0.78% of their DNA on average. That translates to about 53 centimorgans, the unit geneticists use to measure shared genetic material. But there’s a wide range. Some third cousin pairs share as little as 0.06% while others share up to 2.2%, depending on how the randomness of genetic inheritance played out across four generations.
Each time a parent passes DNA to a child, the segments get shuffled and recombined. Over four generations, a lot of shuffling happens. This means some segments from your shared great-great-grandparents made it to both you and your third cousin, while many did not. The result is a small but measurable overlap.
About 98.5% of third cousins share at least one detectable DNA segment. That means roughly 1 in 67 third cousin pairs will share no identifiable DNA at all, even though they are genuinely related through their family tree. This doesn’t mean they aren’t blood related. It just means the genetic evidence has been diluted below the detection threshold after four generations of recombination.
Third Cousins Compared to Other Relatives
To put the relationship in perspective, here’s how DNA sharing scales across different cousin levels:
- First cousins share about 12.5% of their DNA (same grandparents)
- Second cousins share about 3.1% (same great-grandparents)
- Third cousins share about 0.78% (same great-great-grandparents)
- Fifth cousins share so little DNA that only about 32.7% of pairs have any detectable overlap at all
Each additional generation roughly cuts the expected shared DNA in half. By the time you reach fifth cousins, the genetic connection is faint enough that most pairs look unrelated on a DNA test. Third cousins sit in a middle zone: clearly related by blood, but with a modest genetic overlap that you’d never notice without testing.
Can a DNA Test Find Your Third Cousins?
Commercial DNA testing services like 23andMe and AncestryDNA can detect most third cousin relationships, but not all of them. Since 98.5% of third cousin pairs share at least one identifiable DNA segment, the vast majority will show up as matches. The companies’ algorithms then estimate the relationship based on how many centimorgans you share.
The tricky part is precision. Because the range of shared DNA is so wide (0.06% to 2.2%), a third cousin who shares more DNA than average might be labeled as a second cousin, while one on the low end might be categorized as a fourth cousin or more distant match. DNA tests are good at confirming that a connection exists, but the exact relationship label is an educated guess. Combining DNA results with a family tree is the most reliable way to confirm a specific third cousin connection.
Are Third Cousins Considered Close Family?
Genetically, third cousins are distant relatives. Sharing less than 1% of your DNA puts the relationship well outside the range where genetic similarity has any practical medical or biological significance. For comparison, any two people of the same broad ethnic background who aren’t related at all typically share some DNA simply by chance.
Socially, third cousins often don’t know each other exists. Most people can name their first cousins easily, some know their second cousins, and very few can identify their third cousins without doing genealogy research. You likely have hundreds of third cousins, possibly over a thousand, depending on family size across those four generations.
Legally, third cousin marriages are permitted everywhere in the United States and in most countries worldwide. The shared genetic material is too small to raise the kind of concerns associated with closer relative pairings. Genetic counselors generally consider third cousin relationships equivalent to unrelated individuals in terms of risk to offspring.