DNA Relatives finds matches by comparing your DNA with other 23andMe members. When two people share identical segments of DNA, this indicates that they share a recent common ancestor. The length and number of these identical segments is used to predict the relationship between relatives.
23andMe can find relatives on all branches of your family tree using autosomal DNA.
Unlike other genetic genealogy services that identify relationships on only the direct paternal or maternal lines, DNA Relatives can identify relationships on any branch of your family tree. It does this by taking advantage of the autosomal chromosomes—the 22 chromosomes that are passed down from your ancestors on both sides of your family.
For instance, a close relationship such as your paternal grandmother would be invisible to other genetic genealogy services (since she doesn't share your maternal or paternal haplogroup), but would be easily identified by DNA Relatives.
Does DNA Relatives use maternal/paternal haplogroups to find matches?
DNA Relatives does not use your haplogroup to find matches, and not all people who share a haplogroup are listed as relatives. Most of your relatives will actually fall outside of your haplogroup, because your haplogroup only tells you about direct paternal or maternal ancestors. Typically, the DNA mutations that define a haplogroup occurred thousands of years ago, so many pairs of people who share a haplogroup are not closely related.
Once you've found a relative, your haplogroup can help confirm a suspected relationship. For example, a cousin who you believe to be related to you through lines of females originating with a shared great great great grandmother should share your maternal haplogroup.
How many relatives will I find through 23andMe?
The number of relatives will vary for each person and will grow over time as more people join 23andMe. For example, people with European ancestry often have hundreds of relatives. People with Ashkenazi Jewish ancestry, a group that is more related than average, may have up to one thousand relatives. People with Asian or Middle Eastern ancestry will likely have fewer matches.
What determines the predicted relationship?
DNA Relatives estimates a predicted relationship and range using the number of segments and percent DNA shared. In general, for the same percent DNA shared, longer segments give a closer predicted relationship than a greater number of shorter segments. The estimate is based off a number of assumptions about population growth and characteristics of specific populations.
How accurate is the predicted relationship?
The randomness of DNA transmission across generations makes it difficult to pinpoint exact relationships for more distant cousins. We show that uncertainty by providing the relationship range and in these cases the predicted relationship should be treated as a best guess. Closer relationships tend to have a tighter range, reflecting the greater amount of information found in closer relationships. The accuracy of the degree of relationship predicted increases as the relationship gets closer.
Matches are labeled as "distant cousins" when the degree of relationship is difficult to estimate, due to the small amount of DNA shared. For every degree of separation in a relationship, the average percent DNA shared drops by half, so that the percent DNA shared remaining is quite small when you get to distant cousins. The vast majority of relatives found by DNA Relatives share a common ancestor within the last five to ten generations. A few may be more distantly related.
There can be more than one type of relationship that shares the same percentage of DNA. If the predicted relationship is one that reflects the right percent of DNA shared, but isn't what you know the relationship to be, you can select the known relationship instead. You can find a list of the average percent of autosomal DNA shared by close relatives on the International Society of Genetic Genealogy (ISOGG) website.
Because DNA Relatives only identifies people who shares a DNA segment of at least 7 cM (centiMorgans) and 700 SNPs long, you can be confident that the 23andMe members on your list are your relatives.
How can a child have a match that doesn't appear in either of their parents' DNA Relatives?
It does happen that a child sometimes shows a match that's not visible in either parent. In these cases, it's almost always the case that the match is indeed present in one of the parents, but the segment identification algorithm has missed the segment in the parent. This typically happens with short segments, where it is more likely that a candidate segment will fall just under one of the algorithm's thresholds.