You can view the segments of DNA you share with another 23andMe user within the DNA Relatives feature and the Your Connections feature. When you compare yourself to another 23andMe user in either feature, a section appears and displays a diagram of your matching segments of DNA. These segments were likely inherited from an ancestor you both share.
Remember, the genome is organized into 22 pairs of chromosomes, plus the sex chromosomes (two X, or one X and one Y). The Relationship diagram only shows one horizontal line to represent each chromosome pair. We use different colours to show whether you share matching DNA segments on only one copy of a chromosome (inherited from only one of your parents), or on both copies (inherited from both of your parents). The colour of the shared segments show whether one copy is identical (i.e. half-identical), or both copies are identical (i.e. completely identical) between two people.
If two people are related within the last 10 generations or so, we expect them to have shared DNA across relatively large stretches of DNA — that is, more than would be expected by chance.
In this article, we will review how 23andMe detects matching segments of DNA and explore how certain relationships will likely appear in this feature:
How to Compare with Other 23andMe Customers
You can compare your results with other 23andMe customers in a couple of ways. Depending on the privacy settings of both you and the second individual, you may find one comparison method preferable.
The 23andMe Your Connections feature allows you to connect with other 23andMe customers and compare your results without participating in the DNA Relatives feature. In order to compare with another individual in Share and Compare, you will need to be sharing reports with that individual. Once you have established a sharing connection, click the name of the individual to compare results.
Example Data Only
The DNA Relatives feature is an interactive 23andMe feature that helps you find and connect with your genetic relatives and learn more about your family story. Participation in the DNA Relatives feature is optional and you must opt in to the feature in order to see your genetic relatives or be seen by your genetic relatives.
If you and the other individual are both participating in DNA Relatives and you have established a sharing connection (look for the blue dot in the Sharing column) or the match is participating in Open Sharing (look for the purple dot in the Sharing column), you can view your shared DNA segments from within the DNA Relatives feature by clicking on his or her name in your list.
Example Data Only
DNA Relatives - Open Sharing
The DNA Relatives feature is an interactive 23andMe feature, allowing you to find and connect with your genetic relatives. Participation in the DNA Relatives feature is optional and you must opt in to the feature in order to see your genetic relatives or be seen by your genetic relatives. This feature has multiple privacy options to suit your individual preferences. For the most visibility, you can choose to participate in Open Sharing. Open Sharing makes select genetic information — including your full profile name, overlapping DNA segments, and Ancestry Composition Report — available to your matches in DNA Relatives without an individual sharing request.
You can view your shared DNA segments with any individuals in your list who are participating in Open Sharing by clicking on their names from within the DNA Relatives feature.
DNA Comparison Feature
If you would like to view shared DNA segments between multiple matches at once, use the DNA Comparison feature. To select an individual to compare with, you must either be sharing reports or they must be participating in Open Sharing.
Example Data Only
Detecting Matching Segments of DNA
We can detect stretches of shared DNA by comparing Single Nucleotide Polymorphism (SNP) genotypes and looking for mismatches. If a continuous region of matching SNPs is longer than a minimum threshold, which is set to avoid chance matches, then it is likely to have been inherited from the same ancestor.
It is possible for our algorithms to miss short identical segments or identify short matched segments even if no common ancestor exists. As scientific knowledge improves and and we improve our algorithm over time, our estimates will become more precise.
The Relationship comparison colours a segment as pink if the segment is half-identical, which means you and the other person have matching DNA on only one copy of a chromosome. A segment is purple if it is completely identical, which means you and the other person have matching DNA on both copies of a chromosome. If two people are closely related, we expect them to have shared DNA across relatively large stretches of DNA — that is, more than would be expected by chance. To see an example of how a relationship might appear, select the relationship below:
Parent to a son or daughter
Children inherit 50% of their genomes from each parent, so if you compare a mother and daughter you will see that they are half-identical across the entire genome (as designated by the pink color). In this case, the common ancestor is the mother herself.
Grandparent to a granddaughter or grandson
Because a parent’s chromosomes recombine before being passed down to his or her children, a child receives a mixture of his or her grandparents’ genomes. For example, the DNA a mother passes on to her son is a mixture of DNA from the maternal grandmother and the maternal grandfather. Only patches of a grandchild’s genome will be half-identical (as designated by the pink color) to each grandparent’s genome. In our example below of a grandson compared to his maternal grandfather, you can see that about half of the chromosomal regions are half-identical.
Siblings (identical, full siblings and half siblings)
Full siblings will share completely identical (as designated by the purple color) and half-identical segments (as designated by the pink color). We can tell where the siblings share regions of complete identity — that is, stretches of the genome where the two siblings match each other on both pairs of the chromosome. In these cases, both siblings inherited the same segment of DNA from both their mother and father.
Half siblings share only one parent, so none of their shared segments will be completely identical. When comparing half siblings, only the pink half-identical segments will be visible.
Identical twins inherit entire chromosomes from the same parents. So in the Family Traits Genome View tool, they would look completely identical (purple) across the entire genome.
Distantly related individuals
Two individuals can carry identical DNA either because by chance they happen to have the same genotypes at a region, or because they inherited that stretch of identical DNA from a recent common ancestor. If a continuous region of matching SNPs is longer than a minimum threshold, which is set to avoid chance matches, then it is likely to have been inherited from the same ancestor.
If two individuals share half-identical segments, it is an indication that the two individuals are possibly related. The degree of relatedness is proportional to the total length of shaded genome; for example, one small chunk of half-identity on a single chromosome would indicate that two people are distant cousins. Generally, many longer stretches of DNA indicate a more recent common ancestor.
Generally, two unrelated individuals will have no completely identical or half-identical stretches in their genomes.