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Viewing Shared Segments of DNA

You can explore the segments of DNA you share with another 23andMe user within the DNA Relatives tool and the Share and Compare tool. A Relationship section appears in both tools when you compare yourself to another 23andMe member and displays matching segments of DNA found between two people — segments likely inherited from a common ancestor.

Remember, the genome is organized into 22 pairs of chromosomes, plus the sex chromosomes (two X, or one X and one Y). The Relationship pictorial only shows one horizontal line to represent each chromosome pair, but we colour them to 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 closely related, 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 tool:

How to Compare with Other 23andMe Members

You can compare your results with other 23andMe members a couple of ways. Depending on the privacy settings of both you and the second individual, you may find one comparison method preferable.

Share and Compare

The 23andMe Share and Compare tool allows you to connect with other 23andMe members and compare your results without participating in the DNA Relatives tool. In order to compare with another individual in Share and Compare, you will need to be sharing results with that individual. Once you have established a sharing connection, click the name of the individual to compare results.

Share_and_Compare_-_Sibling.jpg Share_and_Compare_-_Relationship.jpg 

Example Data Only

DNA Relatives

The DNA Relatives tool is an interactive 23andMe tool, allowing you to find and connect with your genetic relatives and learn more about your family story. Participation in the DNA Relatives tool is optional and you must opt in to the tool 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 compare results from within the DNA Relatives tool by clicking on his or her name in your list. 

DNAR-sharing.png DNAR_-_Relationship__Ron_.jpg

Example Data Only

DNA Relatives - Open Sharing

The DNA Relatives tool is an interactive 23andMe tool, allowing you to find and connect with your genetic relatives. This tool 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 reports — available to your matches in DNA Relatives without an individual sharing request.

If you are participating in Open Sharing, you can compare yourself with another individual in your list who is also participating in Open Sharing by clicking on his or her name from within the DNA Relatives tool.

DNAR-open_sharing.png DNAR_-_Relationship__Lily_.jpg

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.

Example Comparisons

The Relationship comparison colours a segment as pink if the segment is half-identical (one copy is shared), and purple if the segment is completely identical (both copies are identical) between two people. 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
  • Grandparent to a granddaughter or grandson
  • Siblings (identical, full siblings and half siblings)
  • Distantly related individuals
  • Unrelated individuals
Comparing 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 colour). In this case, the common ancestor is the mother herself.

Parent_-_full_vs_half.jpg

Comparing 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 colour) 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.

Grandparent.jpg

Comparing Siblings

Full siblings will share completely identical (as designated by the purple colour) and half-identical segments (as designated by the pink colour). 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.

Sibling_-_Full_vs_Half.jpg

 

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.

Half-siblings_-_full_vs_half.jpg 

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.

Identical_twins.jpg

 

Comparing 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.

distantly_related.jpg

Comparing unrelated individuals

Generally, two unrelated individuals will have no completely identical or half-identical stretches in their genomes.

Not_related.jpg

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