Do siblings have the same DNA? It seems like brothers and sisters should have the same ancestry background. After all, they have the same parents, right? While biological siblings DO have the same family tree — parents, grandparents and so on, their genetic code might be different. Because of how DNA is passed on, it is possible for siblings to have differences in their ancestry at the DNA level.

Culturally they may each say, for example, that they are 1/8 German. But at the DNA level, one may have no German DNA at all. While this might seem strange or even impossible, it’s actually pretty common. It’s a consequence of the complex relationship between genetics, ancestry, and ethnicity.

So how is this possible? The answer is in the process of reshuffling known as genetic recombination. Children inherit half of their DNA from each parent: 50% from Mom and 50% from Dad. The differences between siblings occurs because the inherited DNA is completely random from each parent. The random 50% of Mom’s DNA one sibling might get is different than the random 50% another sibling gets. It’s simply a matter of biology. That’s true even for fraternal twins. The only way siblings will have the exact same DNA is if they are identical twins (formed from the exact same sperm and egg).

Finding out one’s ethnicity breakdown is probably the main reason people use DNA tests like those from AncestryDNA, 23andMe, MyHeritage or other companies. How do they quantify someone’s genetic ancestry? In other words, what does it really mean to say someone has “Irish” or “German” or “Italian” genes? While every person’s DNA is unique (except identical twins), the vast majority of the human genome —over 99%— is identical in everyone. As humans have evolved, random mutations have accumulated in our DNA, causing more variation between our genomes. (We’ve all probably learned a little something about mutations in today’s coronavirus headlines.)

The most common mutations are called single-nucleotide polymorphisms or SNPs (pronounced “snips”). This means that one base of DNA has been changed at a particular location in the genome. If those mutations happen in sperm or egg cells, the altered sequence can get passed to a person’s children, grandchildren, and future generations.

Geneticists have classified millions of distinct SNPs in various locations in the genome. When a group of people becomes isolated (geographically or culturally), new types of SNPs will appear in the population. Over time, these variations will become common within that group but rarely found in others. By analyzing the genomes of many people in different groups, researchers can identify the characteristic SNPs of that group.

DNA tests analyze the SNPs in a person’s genome and compare it to the genetic sample profiles associated with different ethnic groups. Using this information, they can determine someone’s ethnic background and where their ancestors are from.

But there are some good reasons to take the ethnicity result with a grain of salt. Each testing company uses its own proprietary algorithms to calculate this, so that’s why your ethnicity results from AncestryDNA may look different from your 23andMe results. And these tests calculate ancestry by comparing results to databases of known samples. And for now, the majority of samples tend to cluster around North America and Europe. For people from other parts of the world, there aren’t as many points of reference, and results tend to be less specific.

In the graphs above, you can see how the DNA of each succeeding generation becomes more diluted, if you will. When doing genealogy research, that is why it is important to test the oldest generation you can.

For me, the second component of DNA testing is far more important: DNA matches. Many DNA tests compare samples of their users to find possible relatives — a gold mine for genealogical research and breaking down brick walls in your family tree. Connecting with other people who share DNA segments can help in trying to locate or find information about ancestors further back in your tree. Perhaps someone with whom you have a DNA match has documents, photos or info you don’t. When a whole group of people have matching DNA, it likely means they all descend from a common ancestor. That’s when the research gets fun.

And remember, because siblings have different DNA, that means they very likely will have some different DNA matches. I’ve found that my Mom, Uncle Hank and Uncle Dick, all whose DNA profiles I manage, have some DNA matches in common, but more importantly, they each have some distinctly different DNA matches. That helps me piece together all possible connections. I don’t have DNA results for my Uncle Joe or Uncle Dan, who passed away before I was really doing this kind of analysis, but I do have results from Uncle Joe’s daughter, Trish Lewis. And yes, there are people with whom Trish has DNA matches that my Mom, Uncle Hank and Uncle Dick do not. That can only mean that a snippet of the DNA that came down to Trish from her Dad, was DNA that Joe inherited but his siblings did not. So every time I am looking to see if family members match a particular person of interest, I have to remember to check to see if they match Trish.

Below is an example of a DNA match between my mother and a woman from Dallas named Delia Rea. Her Mattimores came from the Nashville area. We haven’t yet figured out our common ancestor in County Roscommon from whom both of our Mattimore lines descend, but clearly we are distantly related. She also shares DNA segments with my uncles Hank and Richard, Trish Lewis and Jane Hashey. Delia and I hope to figure out our common link one day.

So if you’ve ever wondered “Should I bother to take a DNA test if my brother or sister has already taken one?” my answer would be most definitely yes, assuming someone is interested in ethnicity differences or doing genealogy research.

One of the most common questions I get is “Which company should I use for DNA testing?” I use as many as I can as there are usually different DNA matches with each. But for those just starting out or wanting to do only one, I would recommend using the one that most of your other family members use, whatever that may be. That way, your reference points will be the same. In my family’s case, and I believe for other Mattimore cousins, it is AncestryDNA.

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