How Much DNA Do Siblings Share?

October 6, 2021

The fast-rising popularity of ancestry DNA tests has brought more and more curious information-seekers to the internet, where vast databases of ancestral details and regional connections tell at least some family history for millions of people around the world. In the process, users can sometimes find close family relationships they didn’t know. So how much DNA do siblings share?

As with any scientific field of research, genomics (the study of genes) isn’t always exact. Instead, genealogists offer ranges and averages for questions about shared DNA with certain family members and relatives.

In the case of a full sibling, one with both common parents, the average share of DNA is 50%, according to ancestry DNA site 23andMe. This is the same as a parent, although the average shared DNA of a son with his father is 47.5%, and more than the 25% average DNA shared between a grandchild and grandparent or nephew/niece and aunt/uncle. As the name indicates, identical twins share 100% of their DNA. The range for full siblings is anywhere from 38% to 61% in shared DNA.

In centimorgans, defined as a unit used to measure genetic linkage, full siblings share a range of 1,613 to 3,488 centimorgans, or an average of 2,613, according to genealogist Blaine Bettinger’s Shared cM Project, a database with more than 60,000 contributing genealogists. For comparison, people share a range of 2,376 to 3,720 centimorgans with one of their parents.

“Full siblings are very similar (to half-siblings) in that it’s still a range and it’s still a parent giving you their DNA,” said Bettinger, a New York-based genealogist who runs the Genetic Genealogist blog. “But you’re sharing two parents, so there’s a lot more of a chance to match genetic data.”

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Recombination of genes shuffles the DNA deck among siblings

To understand how each person’s genetic makeup comes together, a few terms must first be defined.

DNA, as defined by the National Human Genome Research Institute, is “the chemical name for the molecule that carries genetic instructions in all living things.” In the process of creating new life, the DNA of the parents copies itself to pass on to offspring.

You have also heard of chromosomes. Humans each carry 23 pairs of chromosomes, which are packages of DNA. Each parent contributes one of the two chromosomes in each pairing, helping their offspring take on some of their genetic data. Of those 23 pairs, one of them is a sex chromosome—females carry two X sex chromosomes; males have one X and one Y sex chromosome.

The process of recombination in humans is essentially when DNA from two parents is collected and mixed together to create a child. There is no fixed pattern for the genetic information that is collected from both parents, which is how children can range in the DNA they share with their siblings.

Bettinger likened recombination to pouring two bags of candy into a bowl and mixing them up. If someone reached in and took out half the bowl, the combination of candies would be different from the same experiment conducted a second time using the same bags.

“A parent has two copies of every chromosome, giving different chromosome combinations to their children,” Bettinger said “While the siblings share a large pool of DNA, it’s not the same exact pieces. The father jumbles up his DNA. The mother jumbles up hers.”

Understanding full siblings makes half-siblings simpler to grasp. Instead of pulling DNA from two shared parents, half-siblings pull DNA from one shared parent and a different second parent. This is why the average share of common DNA between half-siblings is 25%, with a range of 17-34%. There is also a type of relationship between full and half-siblings called “three-quarter siblings”.

In centimorgans, half-siblings share roughly 1,160 to 2,436 between them. This is, as you might assume, about half of the centimorgans shared by full siblings.