Learn how testing yourself and your siblings can break more genealogical brick walls!


Happy Sibling’s Day! Did you know your siblings may have inherited DNA you didn’t and testing them can help you break down genealogical brick walls? Let’s take a look at the various ways testing your siblings can help with the various types of tests!

Break Brick Walls and Confirm Family Stories With Autosomal DNA

Autosomal DNA (atDNA) is the 22 pairs of non-sex chromosomes you inherit from your parents. Some atDNA tests, including our Family Finder, also test the X chromosome, which is inherited from your mother; and if you’re female, you also inherit an X chromosome from your father.

atDNA goes through random recombination each time it’s passed down. This means you and your siblings (unless you’re identical twins) inherited a different mixture of your parents’ DNA—in fact, you only share 50% of your atDNA with a sibling!

Source: Pritchard DNA Project (www.pritcharddnaproject.com)
Source: Pritchard DNA Project (www.pritcharddnaproject.com)

Because you didn’t inherit half of what your siblings did, your siblings will probably have some matches and ancestral populations that you don’t.

You and your siblings will share all of your immediate family matches, along with your 1st cousins and typically your 2nd cousins, with each other. However, your more distant cousin matches can differ. In fact, when it comes to your more distant cousins, the more distant the genealogical relationship, the less likely you’ll match due to random recombination.

Information derived and reformatted from Table 1 in the paper
Information derived and reformatted from Table 1 in the paper "The Probability that Related Individuals Share Some Section of Genome Identical by Descent" by Kevin P Donnelly, Statistical Laboratory, Cambridge University, Cambridge, England. Published: Theoretical Population Biology: 23, 34-63 (1983). For more information visit www.isogg.org/wiki/Cousin_statistics

These more distant cousin matches are incredibly useful for breaking down genealogical brick walls and confirming your paper trail—they share common ancestors with you further back in your family tree. You may have built your tree out enough to already have those common ancestors in it. If so, you can confirm your paper trail through DNA matching. If you haven’t built your tree out to that common ancestor, maybe your match has, and you can expand your tree with help from them!

Ancestral Populations May Differ in Siblings

Your siblings will also inherit different ancestral populations (often referred to as “ethnicity”) than you. The more distant you are from an ancestor in a specific population, the less likely it is that you’ll inherit a percentage of that population.

Do you have a family story about your fourth great-grandmother, who was Native American? You may not have inherited any Native American percentages, but your siblings might. By testing multiple family members that share that same ancestor, like you and all your siblings, you may be able to confirm more ancestral populations.

Discover Your Paternal Ancestry with Y-DNA

The Y chromosome is passed down from father to son, meaning that only males inherit a Y chromosome and only males can take a Y-DNA test. Typically, Y-DNA is inherited without mutations. Two brothers will usually match exactly at the Y-111 STR (short tandem repeat) level, meaning their Y-DNA is exactly the same.

Sometimes one will have a SNP (single nucleotide polymorphism) mutation that the other doesn’t, and that can be determined with the Big Y-700′; but more often, they’ll be exact matches at the Big Y-700 level as well.

Testing Y-DNA in Male Family Members

You can test multiple immediate male family members from the same direct paternal line with the Big Y-700 to see if you can uncover unique mutations, but it’s more genealogically helpful if you test one immediate male family member and then more distant male cousins on the same direct paternal line.

You may all belong to the same branch on the Tree of Mankind, or you may form separate branches. The more descendants that are tested (not immediate family members of each other), the more helpful the Big Y-700 results will be in determining mutations that are unique to a specific surname line or unique to different branches of that surname line.

FamilyTreeDNA Y-DNA Direct Paternal Path

You can also use this strategy to test other paternal lines in your family tree. Direct male descendants of your maternal grandfather can be tested to discover the story of that ancestral line, and so on.

There is a situation where testing multiple brothers with Y-DNA is helpful if you aren’t sure if they have the same father. Since Y-DNA is passed down from father to son, if two brothers don’t share the same father, they won’t match on a Y-DNA test.

Testing Y-DNA in Female Family Members

Since only males inherit Y-DNA, if you are female and want to dig deeper in your father’s or grandfather’s direct paternal ancestry, get your brother or a cousin on the direct paternal line a Y-DNA test.

Discover your Maternal Ancestry with mtDNA

Mitochondrial DNA (mtDNA) is passed down from mother to child. Everyone inherits mtDNA, but only females will pass it down to their children.

FamilyTreeDNA mtDNA Direct Maternal Path

mtDNA mutations happen on average about 20 times slower over time than Y-DNA mutations. You and your siblings will likely be exact matches at the mtFull Sequence level, meaning your mtDNA is exactly the same.

Testing mtDNA in All Family Members

Similar to Y-DNA, the advantage with mtDNA comes from testing one immediate family member (you) and more distant cousins on the same direct maternal line. You can also use this strategy to test other maternal lines in your family tree. Direct maternal descendants of your paternal grandmother can be tested to discover the story of that ancestral line, and so on.

The Tree of Womankind

Through The Million Mito Project, FamilyTreeDNA’s Research and Development team is building the largest Tree of Womankind in the world.

FamilyTreeDNA The Million Mito Project

The team is examining an abundance of new test data that has come in since 2016, the last time the tree was expanded. With the release of the new Mitotree, the R&D team will be able to provide more specific haplogroups and more specific information on when you share a common ancestor with your mtDNA matches.

Discover Something New

Whether you’re testing yourself, your siblings, or your cousins, there’s always something new that you can discover about your ancestral story! Confirm your paper trail, uncover new ancestral locations, and break genealogical brick walls by testing multiple family members today.

Headshot of Katy Rowe-Schurwanz - Product Manager at FamilyTreeDNA

About the Author

Katy Rowe

Product Manager at FamilyTreeDNA

Katy Rowe has always been interested in genealogy, inspired by her maternal grandparents, who told her stories about their family and family history when she was little. After studying anthropology and history in college, she joined FamilyTreeDNA in 2015 and became the Trainer for Customer Support. Katy created and improved training processes and was fundamental in the creation of the Big Y Specialist team. In September 2021, she became Product Manager and has focused closely on improving FTDNA’s genetic genealogy products.

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