More people than ever are using DNA testing to help them find relatives. If you believe you have half-siblings that you’ve never met, genetic testing might help you identify a relationship. The test is a great place to begin your search for long-lost relatives because it helps you find DNA links.
No. DNA is unique to each individual unless he or she is an identical twin. However, DNA testing can help you identify siblings and half-siblings because you share more DNA with those people than with people not related to you. But simply sharing DNA with someone doesn’t necessarily mean that person is a sibling. Some sibling DNA, especially when people are half-siblings, is the same as 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 chromosomes – one from your mother and one from your father. Most of us have 46 total chromosomes in the body.
Scientists numbered chromosomes 1 through 23 to make them easier to locate and understand. The 23rd pair is significant because it establishes biological sex. Females (XX) inherit X chromosomes from both parents, while males (XY) inherit a Y chromosome instead of an X from their fathers.
Biological siblings with the same parents share about 50% of their DNA.
You got about half of your DNA from your mother and your brother or sister got half of his or her DNA from the same mother. This means you and your full biological sibling share half of 50% of your mother’s DNA. This is 25%. If they share the same father, they inherited about 25% of their father’s DNA and share 50% total – half from mom and half from dad.
Yes, absolutely, but not exactly the same. Siblings have some of the same DNA and more of the same DNA than more distant relations. The same is true for sisters and sister and brother pairs. All share the same DNA, but it varies to some degree in the total amount.
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 image 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 the half 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 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% - the amount of DNA you share with a half-sibling contributed by your shared parent.
Now, keep in mind, 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.
DNA testing can help show the potential for a sibling relationship, but it won’t prove it with certainty.
As a matter of fact, it’s as likely that a person is your half-sibling as it is that they are your cousin. Half-siblings also have a similar genetic presentation as grandparents and aunts and uncles. The best DNA testing can narrow down the relationship better than basic tests, so if you are relying on an at-home test to find a half-sibling, you’ll likely need to dig deeper to prove the relationship.
At-home tests are a great place to start, and then you can follow up by adding known family members and conducting additional research.