Your siblings’ DNA tests can help your family history–even if you have already tested! Here’s what you can do with the combined power of your brothers’ and sisters’ DNA test results.

How many DNA tests does one family need? Does a genetic genealogist gain any advantage by testing their siblings, even though you all come from the same parents? It might seem counterintuitive, but the bottom line is YES!–if both of your parents aren’t likely to take DNA tests themselves.

Stop and consider for a minute what YOUR DNA test reveals about your parents. Your DNA report is (on average) a 50% mish-mash of your parents’ DNA. Half of your dad is in you, and half of your mom. If you have any full siblings, your parents also gave half of their DNA to those brothers or sisters, but it’s a different half than the half you received. We’ll come back to this idea in more detail in a minute. The main idea to know right now is that your siblings’ DNA provides additional insight into your parents’ heritage.

So what can you actually DO with your siblings’ DNA test results, separately or together? I’m glad you asked. Before I offer 3 tantalizing answers, here’s a quick reminder. When you invite anyone to take a DNA test, be sure they know they might learn new (and perhaps unwelcome) information. For example, that they’re not biologically connected to one or more relatives or that they have biological relatives they didn’t know about. People shouldn’t test unless they consider themselves prepared for those possibilities.

What can you do with your siblings’ DNA tests?

1. Get a second (and third) opinion on ethnicity results

The formulas used to calculate your ancestral ethnicity are incredibly complex and involve a degree of latitude. If you test YOURSELF twice with the same company, you’ll likely get slightly different results. Knowing that full siblings inherit randomly-different parts of their shared parents’ DNA, you would fully expect variation between you and your siblings’ ethnicity percentages.

Intrigued? Confused? Get a free guide to DNA ethnicity

That said, it can be interesting (if not always statistically meaningful) to see how your siblings’ ethnicity results compare against your own. Let’s look at this example of 3 siblings who tested with AncestryDNA:

Take a moment and imagine that you are Sibling 1, then that you are Sibling 2, then that you are Sibling 3. How would your outlook on your heritage change based on the results of your DNA test? How would having the additional information from your siblings change that perspective?

Perhaps Sibling 3 would feel the most powerful difference: he’s less Scottish and more Scandinavian than his siblings. Siblings 1 and 3 would likely disregard such a tiny Irish percentage, but Sibling 2 inherited 7%, so maybe there was an Irish ancestor after all. 

I get questions about this process with siblings sometimes. Like: “We have documented Native American ancestry. My 5th great-grandmother was 100% Ojibwe, so I wasn’t surprised when my ethnicity results showed 6% Indigenous North America. My full sister’s DNA test didn’t show any Native American ancestry at all. How can this test be accurate?”

This is a good question. But it seems to presume that our ancestors’ DNA gets blended up and handed to us like some sort of chromosome smoothie. That’s not the case. DNA recombination works more like a chef’s knife and produces something more like a salad, where each of us gets helpings with different amounts of greens, carrots, and croutons. That’s why peeking into your siblings’ DNA salad can better help you understand the original ingredients in the big salad bowl of your parents’ genes.

2. Find more DNA matches

When you have more of your parents’ DNA through your siblings, you can expect to find DNA cousins who don’t show up in your match list because you simply did not inherit shared DNA, but your sibling(s) did. Remember, you only share DNA with about 50% of your 4th cousins. So testing your siblings will certainly find more 4th cousins.

Additionally, your siblings may match your matches at a higher or lower level than you do. A match who matches your sibling at a significantly higher level than you may actually be more closely related to your family than you may have predicted based solely on your DNA connection to them. Check out this example of using multiple siblings to figure out a relationship to a mystery DNA match.

These new matches may allow you to make new connections with previously unknown relatives by expanding your family’s pool of DNA matches. You should all have the same first and second cousin matches. About the level of a second removed cousin is where you may start to see some differences.

To make these comparisons, you’ll need to be able to see your siblings’ full list of matches, and how much DNA they share with your matches. So hopefully they’ll grant you that access. (Even if they don’t, as long as you tested at the same companies, you can at least review your shared matches with each of them. At MyHeritage, you can also see how much DNA your siblings who test are sharing with your shared matches (you can do this at 23andme too if the sibling has public settings.)

If you and your siblings have already tested at different companies, you’ll have the benefit of each reaching different pools of test-takers. But it’s also good to compare your matches at the same testing company, so consider testing together at AncestryDNA or 23andMe, or transferring your results to the companies that accept transfers (MyHeritage and Family Tree DNA).  

3. Reconstruct the DNA of Your Parents

WHAT?! Yes. You read that correctly.

By combining multiple siblings’ DNA test results, you can reconstruct more of your parents’ DNA than the 50% that comes by just examining your DNA alone. In fact, there’s a formula that predicts, on average, how much of the parents’ DNA will be represented when you combine multiple siblings: 1-(½)s (where s=number of siblings). 

Good news: You don’t have to memorize the formula–or even use it. You can just look at this handy table. 

As you can see, by adding the DNA test of just one sibling, you bump up the access to your parents’ DNA from 50% to 75%. With three total siblings, the average amount of parental DNA represented goes up to 87.5%. By the time you get up to 6 or 7 siblings, on average you can reconstruct essentially 100% of mom’s and dad’s DNA.

As you can see, collaboration with siblings gives you A LOT more of your parents’ DNA to work with! (These figures represent averages because sometimes a child will inherit more than 50% from one parent and less from the other.)

What’s the point of reconstructing your parents’ DNA? What can you DO with it? The answer is cool, but is a subject for another article. Meanwhile, why not brush up on those DNA ethnicity results? You’ll have more talking points that might help your siblings decide they’re ready to test.

Get our free guide to DNA ethnicity