In This Article
In This Article
More people than ever are using DNA testing to help them find their relatives.
If you believe you have half-siblings that you’ve never met, genetic testing might help you find them. These tests are a great way to look for long-lost relatives because they help you find DNA links.
DNA testing can help show the potential for a sibling relationship, but it won’t prove it with certainty.
In fact, half-siblings may actually share the same amount of DNA with you that a cousin, grandparent, aunt, or uncle do--so there's an absolute way to be sure.
More comprehensive DNA testing can narrow down the relationship better than basic tests, so if you're relying on an at-home test to find a half-sibling, you’ll need to dig deeper to prove the relationship.
Half-siblings share about a quarter (25%) of their DNA. Depending on how DNA is passed down from parents to children, some half-siblings will have more in common while others have less.
This difference in shared DNA is more prominent when comparing half-sisters to sister-brother sibling pairs. Half-sisters who share a father will have more DNA in common than a half-brother and half-sister because of how the X and Y chromosomes are passed down.
Consequently, half-sisters who share a father will share an X chromosome but their brother won't. Half-sisters will always have more DNA in common than they do with a half-brother.
A standard sibling test compares two possible outcomes, known as hypotheses.
For instance, two individuals who are aware they share the same mother might ask a DNA testing company to verify that they share the same father as well.
In this scenario, the DNA testing company evaluates hypothesis 1 (Jane and Joe are full siblings) against hypothesis 2 (Jane and Joe are half-siblings sharing the same mother but with different fathers).
The usual outcome is a probability of about 99% of hypothesis 1: Jane and Joe are full siblings, and 0.07% for hypothesis 2: Jane and Joe are half-siblings with different fathers.
Scenario 2: half siblingship
If two people are aware that they have different mothers but are uncertain whether they share a father, the DNA testing company will perform half a sibling test.
First, analysts will test hypothesis one.
Jane and Joe are half-siblings, sharing a father but having different mothers; against hypothesis 2: Jane and Joe are unrelated, i.e., they do not share a father or mother.
If Jane and Joe are unrelated, the usual outcome will be a probability of 1.2% for hypothesis 1 and 90% for hypothesis 2.
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After you take an Autosomal DNA test (A DNA test that examines your autosomal chromosomes, which contain DNA segments you share with everyone you're related to), your testing company will compare your results to others in its database.
If it finds significant similarities to other people who have taken the same genetic test, you'll see them in your list of matches.
If your ancestry is in a well-studied region like Europe, you may have thousands of matches. While most will be only distantly related to you, a few will likely be close matches.
DNA is unique to each individual unless he or she is an identical twin. However, siblings and half-siblings share more DNA with you than with people not related to you---and you can determine that through DNA testing.
But simply sharing DNA with someone doesn’t necessarily mean that person is automatically a sibling.
Some sibling DNA, especially when people are half-siblings, resembles that of more distant relatives, such as cousins or aunts and uncles.
To understand this better, it helps to have a clearer picture of how DNA works.
DNA is packed into cell units called chromosomes. Each cell in your body contains two copies of chromosomes---one from your mother and one from your father. Most of us have 46 total chromosomes in the body.
These chromosomes are numbered 1 through 23. The 23rd pair is significant because it establishes biological sex.
Females (with XX chromosomes) inherit X chromosomes from both parents.
Males (with XY chromosomes) inherit a Y chromosome instead of an X from their fathers.
It can be challenging to figure out the percentages and imagine what exactly is contributed by a sibling pair’s mother and father.
It helps to imagine the process of passing on DNA with a deck of cards in mind. Shuffle the 52 cards in the deck and lay 26 of them face up. The cards you see represent half of your mother’s DNA that you inherited.
Now put the entire deck back together, reshuffle, and lay 26 cards face up again to represent your full sibling’s DNA. Chances are good you aren’t going to get the exact same 26 cards, but some of them will be the same.
Both you and your sibling each got 26 out of your mother’s 52 cards/DNA, but you didn’t get the same 26.
To determine the total amount of DNA shared between you and a full sibling, you’d repeat this exercise for your father’s DNA. But if you are only half-siblings, you’d only do it once, for your mother or your father---whichever parent you share.
The cards that match when you lay out 26 for each of you represent about 25% of your DNA, which is the amount of DNA you share with a half-sibling.
These percentages vary from person to person. You aren’t going to get a match of 13 cards each time, though it will happen occasionally. The 26 chromosomes we get from each parent are random.
Theoretically, you could have them all match or have none of them match with a sibling. So while the “rule” is siblings share about 50% of their DNA and half-siblings share about 25%, they don’t have to share any at all.
We inherit approximately 50% of our DNA from our mothers and 50% from our fathers, but that’s where “exact” percentages end.
So not only can a brother and sister have different DNA, most do, but it’s in random varying amounts.
Biological siblings with the same parents share about 50% of their DNA.
You and your siblings inherit about half your DNA from your mother and the other half from your father.
This means you and your full biological sibling share 25% of your DNA from your mother. If they share the same father, they have about 25% of their father’s DNA. In total, you and your biological siblings have 50% of your parents' DNA.
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