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Commonly Asked Questions:
Relatedness
Looking for more? Browse through our archive of relatedness/ancestry articles .
How do paternity tests work?
Click here to learn what the markers mean, how they are inherited, and why it's easy to detect parent/child relationships.
What is the best test to tell if two people are related?
Ancestry tests will give you the clearest answer. These tests are very sensitive, and able to detect even distant relatives ! Paternity tests are very accurate for parent/child relationships. But sibling/avuncular are not as good as AncestryDNA or 23andMe at detecting different relationship types.
Why don't I look like my parent?
Physical appearance is complex . Most children aren't mini versions of their parents ... and sometimes they look quite different!
What's the difference between shared centimorgans and shared DNA segments?
A shared segment is a piece of DNA that is shared between two individuals. The length of a segment is reported in centimorgans, which is a unit of measurement. Read more about centimorgans vs segments .
How can I tell if my sibling and I share both parents, or just one?
While both full and half siblings share DNA, full siblings will have stretches of completely identical DNA that is easily detected by ancestry tests. The DNA of full vs half siblings looks different in 23andMe . You will also get an easily interpreted result from AncestryDNA, though it is easiest to load your data into GEDmatch to compare sibling types .
Why do the results from my genetic test give me an inexact percentage?
While you would share on average 50% of your DNA with a full sibling (or 25% with an aunt, 12.5% with a cousin, etc), the actual amount of shared DNA will vary . Check out this interactive tool for shared DNA to understand what the range of shared DNA may be: Shared cM Project
Why do I share 50% of my DNA with a full-sibling, but only 25% with a half sibling?
Click here to learn about heredity, how much DNA you inherit from each parent, and how that relates to % relatedness.
How is it possible that siblings share 50% DNA, while humans and chimpanzees are 98% alike?
There are many ways to compare two sets of DNA ! All humans are actually very, very similar at the DNA level. There are only a handful of differences between any two humans -- so sometimes it's easiest to compare what percent of those spots are different between two people.
Can you calculate shared DNA in more complicated families? Like ones that intermarry, or with incest?
While the actual amount of shared DNA will vary , you can calculate the theoretical amount of shared DNA for any relative type. This article walks through an example of how to do that.
Is it possible to detect incest in a genetic test?
It may be possible , but you probably won't be able to say conclusively unless you look at the parents' DNA as well.
What genetic information does a father pass on?
You get half of all your DNA from your dad! Dads pass on much more than just a Y chromosome.
How much DNA do you share with each grandparent?
You’ll share around 25% DNA with each grandparent , but the exact amount may vary.
What are the risks in marrying a cousin?
The risk in marrying a relative is lower than many people think . Children of first cousins are only 2x more likely to be born with a birth defect than children of non-related individuals. And the risk for children of second cousins is similar to that of unrelated individuals.
Ancestry
How do ancestry tests work?
Ancestry tests compare your DNA to a set of people with known ancestries. Your ancestry is then assigned based on how similar you are to those people. But this means they are limited by who is in the database! If no one from Croatia is in the reference data, ancestry from there will not be detected. Read more on how ancestry tests work .
My ancestry percentage doesn’t make sense! Why did I get this number?
The ancestry DNA percentages aren't completely solid . There are a few things to keep in mind:
Instead of viewing it as "I'm 25% Italian", you should think of it as just saying that you have Italian ancestry.
It can be difficult for these test to distinguish between closely related groups, so your "Italian" ancestry might just show up as "Southern European."
Even though you may have an ancestor from Italy, they might have had Greek, African, or Spanish ancestry themselves.
A very small percent value may not indicate true ancestry.
Your DNA ancestry does not need to define your cultural ancestry.
Why don’t my sibling and I have the same ancestry results?
Siblings often have slightly different results . Even though you both have 50% of your DNA from mom and 50% from dad, it may not be the same 50%.
Why don’t identical twins have the same ancestry results?
Even people with the same DNA can get different results ! This is due to how the algorithms in the genetic tests define ancestry.
Why are mitochondrial DNA and the Y chromosome good for detecting distant ancestry?
Mitochondial and Y DNA are inherited differently than other types of DNA, and do not change very much between generations. This makes them particularly useful for detecting ancient ancestry.