As ancestry and genealogy tests continue their massive rise in popularity across the globe, questions abound for the most curious DNA detectives among us. One commonly asked question: How much DNA do cousins share? Let’s dive into the details.
How much DNA do you share with your cousins?
To answer the question, let’s start with a definition: DNA is essentially our cellular identification code—molecules that make up our extremely intricate genetic information. No single person is genetically the same, but we do share enough similarities with many of our relatives that DNA tests can typically find our familial relations. Naturally, you share the most similar genetic makeup with your parents, though you combine and mix their DNA to create your own gene profile.
The key to your genetic makeup is the chromosome, a combination of protein and DNA. We have 46—or 23 pairs of—chromosomes, half of which are passed down by our father and half from our mother. Only one of those 23 pairs is related to gender. One single chromosome can reveal a seemingly endless supply of genetic information about you.
Think of it this way: Your mother received genetic information from her mother and father, and your father received genetic information from his mother and father. Their parents mixed DNA from their parents, and so on, making the recombination process (the mixture of your parents’ DNA) a tapestry of genetic data. Because of that generation-by-generation, person-by-person mixture, your DNA similarities to your relatives dissipate the deeper into the family tree you get.
How much DNA do first cousins share? Genealogists estimate first cousins share, on average, about 12.5% of their DNA. This is because first cousins have the same grandparents but different parents, with one parent of each cousin being siblings. As with any average, there is a range that comes with it: First cousins may share anywhere from 7.3% to 13.8% of their DNA, according to 23andMe.
“There is so much confusion about cousins and their definitions—I hear about that every day,” said CeCe Moore, chief genetic genealogist for Parabon Nanolabs. “Even though it’s simple, (which level of cousin you are) is very important if someone is going to do genetic ancestry.”
Defined as cousins who have the same great-grandparents, second cousins average roughly 3.1% of shared DNA. This, Moore said, is a significant step up from second cousins, once removed, who are relatives who share the same great-grandparents but are a generation apart. That relation shares just 2% of genetic makeup.
“It doesn’t just halve when going from first cousin to second cousin to third cousin; it’s actually cut four ways,” Moore said. “‘Once removed’ is where it gets halved.”
Cousins who share great-great-grandparents share around 0.8% of their DNA on average. That range is anywhere from 0% to 2.2%, according to 23andMe.
There is a good chance—90%—that third cousins share enough genetic makeup for ancestry DNA tests to find matches, according to Family Tree DNA.
The further apart the relationship on the family tree, the less and less genetic makeup you share. The same Family Tree DNA data mentioned above, cited by the International Society of Genetic Genealogy, indicates only a 50% chance of finding enough genetic information to match fourth cousins.
Fourth cousins, what we often call “distant relatives,” share great-great-great-grandparents. They average 0.2% of shared DNA.
How much DNA indicates incest in family lines?
As ancestry DNA tests grew in popularity, a study from Baylor University revealed the potential for some unexpected results. That is, the discovery of incest along one’s family tree. Typically, the indication of incest lies in shared DNA greater than 50%—the child born out of incest bears either the shared DNA of a parent-child or siblings while also being first cousins with their parents.
There is great medical risk to incest that goes beyond any legal, moral or societal concerns. As Baylor’s study showed, children born out of incest face significantly higher odds of suffering from the rare conditions their parents battled because of the doubling down of similar DNA.