Being cousins generally implies a familial relationship, but it is possible for two people to be considered cousins without actually sharing any DNA. The amount of DNA two cousins share depends on what type of cousins they are and how many generations removed they are from their most recent common ancestor.
What Makes Two People Cousins?
Two people are cousins if they share a common ancestor such as a grandparent or great-grandparent. The closer the ancestor is generations-wise, the more DNA the cousins will have in common. There are many different levels of cousin relationships:
- First cousins share a grandparent and are two generations removed from their most recent common ancestor.
- Second cousins share a great-grandparent and are three generations removed from their most recent common ancestor.
- Third cousins share a great-great-grandparent and are four generations removed from their most recent common ancestor.
- Fourth cousins share a great-great-great-grandparent and are five generations removed from their most recent common ancestor.
The more generations back you have to go to find the common ancestor, the less DNA the cousins will share. While first cousins typically share 12.5% of their DNA, seventh cousins would share less than 1% of their DNA, and cousins related through even more distant ancestors may share no detectable DNA at all.
Why You Can Be Cousins Without Sharing DNA
There are a few scenarios in which two people could be considered cousins but not have matching DNA:
- Distant cousin relationships: The more distant the cousin relationship, the less DNA the cousins will have in common. Seventh, eighth and ninth cousins and beyond might not share any detectable DNA, but would still be considered cousins by ancestry.
- Adopted relatives: If someone is adopted into a family, they become part of that family legally and socially but do not share genetic ties. An adopted person would be considered a cousin to their adoptive family members while not being biologically related.
- Step-relationships: Step-cousins share a family relationship through the marriage of their parents, not through blood. Similarly, someone could be a cousin by marriage without genetic ties.
- Half-relationships: Half-cousins share just one grandparent instead of both. They would share less DNA than a full cousin pair.
In each of these examples, while the cousins have a familial connection, they either share little or no overlapping DNA that would identify them as genetic cousins.
DNA Connections Between Different Cousin Types
While DNA connections can vary greatly between more distant cousins, here is an overview of the typical amount of DNA shared between first cousins through fifth cousins:
| Relationship | Amount of DNA Shared |
|---|---|
| First cousins | 12.5% |
| Second cousins | 3.125% |
| Third cousins | 0.781% |
| Fourth cousins | 0.195% |
| Fifth cousins | 0.049% |
As shown, the amount of DNA shared drops off quickly beyond first cousins. Fifth cousins only share about 0.05% of their DNA, an amount that may not even be detectable with standard consumer DNA tests.
DNA Testing to Confirm or Disprove Cousin Relationships
Genealogical DNA testing can be used to either confirm or disprove suspected cousin relationships, even out to distant cousin connections. Here’s an overview of how DNA testing can help identify cousins:
- Autosomal DNA testing: Autosomal DNA tests like those from 23andMe, AncestryDNA and MyHeritage DNA provide overall percentages of DNA shared, allowing you to compare your DNA against suspected cousins. The higher the shared DNA percentage, the closer the relationship.
- DNA matching: DNA databases can be used to find genetic matches between cousins. Closer relationships generally share longer segments of matching DNA.
- Triangulation: If multiple cousins match the same segment of your DNA, it can confirm a shared ancestor.
- X-DNA: Since X-DNA is passed down more consistently, it can help identify cousin connections through shared X-DNA segments.
DNA testing is the only way to conclusively prove a biological cousin relationship even if your genealogical research suggests you may be related. With more distant cousins, the shared DNA percentages and segments get smaller, making genetic ties more difficult to verify. But for closer cousins, having even small segments of shared DNA can serve as evidence you share a common ancestor.
Legal and Social Definitions of Cousins
While DNA can reveal biological connections between cousins, legal and social definitions also impact who is considered part of your extended family. Things like adoptions, marriages, step-families and estrangements can result in familial relationships not based on blood.
Some key points on non-genetic cousin bonds:
- Adopted relatives become full legal members of the adoptive family.
- Step-cousins are cousins through marriage rather than blood/DNA.
- Half-cousins only share one grandparent in common rather than two for full cousins.
- Family estrangements can result in cousin connections being broken despite DNA ties.
While DNA testing can reveal biological cousins, it is important to remember that families are defined in other ways as well. Who you consider a cousin may be more based on familial bonds rather than precise percentages of shared DNA.
Conclusion
In summary, it is absolutely possible for two people to be cousins without detectable DNA connections. The more distant the cousin relationship, the less DNA overlap there will be. Social and legal definitions of family also impact who is considered a cousin, even if biological ties cannot be proven. While DNA testing can reveal evidence of cousin connections, it does not define on its own who you call your cousins within your extended family.