by Meryl Schumacker, CG
In a toolbox, each tool serves a specific purpose. If you venture beyond a tool’s purpose—try to drive a screw into a piece of wood with only a tape measure—it’s not going to go well. Genealogy’s newest, shiniest toolbox—DNA—is no different. Today, we will parse the DNA-screw from the DNA-tape measure by identifying which DNA tests are the proper tools for which types of genealogical research.
Autosomal DNA (atDNA)
Humans have 22 pairs of autosomal chromosomes, along with a 23rd set of chromosomes that define biological sex (e.g., XX, XY). We inherit our autosomal DNA from both biological parents. Our parents inherited autosomal DNA from their biological parents and so forth, across all branches of our family tree, going back several generations. From our distant ancestors—beyond, say, our fourth or fifth great-grandparents—we may have inherited little to no DNA due to a process called recombination.
To understand recombination, imagine your biological parents’ autosomal DNA like a smoothie: put all of the ingredients into the blender, mix, and pour out about half of the mixture—your autosomal DNA “smoothie.” Due to the randomness of the blending process, your smoothie may have slightly more blueberry than banana. Similarly, when you inherited a random combination of about 50% of each of your biological parents’ autosomal DNA, you may have inherited more DNA from some ancestors than others.
To account for this variation, autosomal DNA is considered most useful for genealogical problems within 5 to 6 generations from the test-taker, or within the last 250 years or so. Tests of autosomal DNA are offered by all of the major DNA-testing companies.
Both males and females have X-DNA. Females have two X chromosomes, one inherited from each biological parent. Males inherit one X chromosome from their biological mother. X-DNA is included in most autosomal DNA tests; check with individual testing companies to confirm.
Like autosomal DNA, X-DNA undergoes recombination and is most useful for research problems from approximately the last 250 years or so. Because biological fathers pass on their X chromosome to daughters and not sons, X-DNA follows a unique inheritance pattern. X-DNA can provide valuable data, but only if the target ancestor passed X-DNA down to the test-taker. Several genetic genealogists and bloggers have designed helpful charts that illustrate X-DNA inheritance.
Mitochondrial DNA (mtDNA)
Both females and males carry mtDNA, but only females pass on mtDNA. This creates an unbroken line of mtDNA, following the biological mother’s mother’s mother’s ancestry, and upwards. mtDNA is “slow-evolving,” which means that it changes very little from one generation to the next, if at all. As such, mtDNA can be used as evidence in genealogical cases that go back hundreds or even thousands of years. mtDNA has been used to track ancient humans’ migrations across the world. mtDNA’s strength—its longevity—is also its weakness. Because mtDNA evolves so little, test-takers and their matches may share common maternal ancestors who lived many hundreds of years ago, beyond the reach of most family trees. mtDNA tests are therefore less popular than other types of genetic tests.
Y-DNA is passed from a biological father to all male offspring, and from those male offspring to their male offspring, creating an unbroken line of shared Y-DNA. Female offspring do not have Y-DNA.
Y-DNA is fairly slow-evolving, though not quite as slow-evolving as mtDNA. This makes Y-DNA useful for a range of genealogical problems, both recent and distant. Due to its inheritance pattern, Y-DNA is only applicable to a male test-taker’s biological father’s father’s father’s line, along an unbroken line of Y chromosome-carrying ancestors. Common ancestors can be easier to identify because Y-DNA matches may share surnames. Females interested in their biological fathers’ Y-DNA cannot test themselves, but can test Y-carrying siblings, paternal uncles, or cousins. Testing requires a Y-DNA test.
All types of DNA have been used as evidence in a range of genealogical cases. The below examples may give you some ideas for how to apply DNA to your research.
Sample Problem: Identifying King Richard III
A skeleton rumored to be King Richard III was famously exhumed from an English parking lot in 2012. Researchers relied upon mtDNA testing to identify the remains as the former king. mtDNA’s slow-evolving traits made them ideal for this research question from the very distant past; King Richard III died in the year 1485. Since the king has no living descendants, researchers tested descendants of his sister. Visit the University of Leicester’s website to read how scientists tackled the case.
Sample Problem: Research in the 18th-19thCenturies
Autosomal DNA, X-DNA, and Y-DNA are useful as evidence in 18th- and 19th-century research problems:
- When faced with minimal paper records, former NGSQ editor Thomas W. Jones used autosomal DNA to identify the biological father of his research subject. Melissa Johnson deconstructed Jones’s work in a previous edition of NGS Monthly.
- Following analysis of Y-DNA, Worth Shipley Anderson identified the father of John Stanfield. Traditional records also identified Stanfield’s mother. This research was published in the June 2018 issue of the NGSQ.
- In the December 2018 issue of the NGSQ, Jill Morelli used a combination of autosomal DNA and X-DNA to identify the biological father of Marie Evelyn “Molly” Frisch.
Sample Problem: Unknown Parentage
Adoptees and children of sperm or egg donors are among those who may use DNA to identify their biological parent(s). Close DNA matches—ideally second cousins or closer—aid the process significantly. To increase the odds of finding a close DNA match, first test autosomal DNA with multiple companies. To lower the total cost of testing, upload 23andMe or AncestryDNA results to FamilyTreeDNA and MyHeritage for a nominal fee.
This article barely skims the surface of DNA and its uses in genealogy. Advance your knowledge by exploring resources like these:
- The International Society of Genetic Genealogy (ISOGG) Wiki offers free encyclopedia articles on everything from haplogroups to X-DNA inheritance.
- When I started learning about DNA, I read Genetic Genealogy in Practice by Blaine Bettinger and Debbie Parker Wayne. I particularly liked that I could test my knowledge with the book’s practice exercises (the answer key is included).
- The upcoming NGS Conference in St. Charles, Missouri, has an entire track dedicated to DNA. There are sessions targeting every level, from beginner to advanced.
- Practice! Take a DNA test and dig into your own results.
Hire a Professional
Consider hiring a professional to help you with your DNA-focused research. Genetic genealogists have different areas of specialty; some solve primarily 18th-19th century cases, others work unknown parentage cases, and some do both (or something else entirely). Understanding DNA’s relationship to your research problem can help you to select a professional whose expertise matches your particular needs.
 As a general vocabulary note, the terms “male” and “female” in this article refer to the biological sexes determined by the process of chromosome inheritance (X and Y chromosomes for males, two X chromosomes for females). Biological sex determined by chromosome inheritance and gender are distinct. Although we may gather evidence of our ancestors’ biological sexes, researchers acknowledge that most evidence cannot convey how our ancestors identified with regard to gender. As such, genetic genealogists use the terms “male” and “female” to refer only to the biological sexes associated with chromosome inheritance; these terms make no assumptions of gender identity or expression.
 Thomas W. Jones, “Too Few Sources to Solve a Family Mystery? Some Greenfields in Central and Western New York,” National Genealogical Society Quarterly 103 (June 2015): 85-103.
 Worth Shipley Anderson, “John Stanfield ‘as he is cald in this country’: An Illegitimate Descent in Eastern Tennessee,” National Genealogical Society Quarterly 106 (June 2018): 85-101.
 Jill Morelli, “DNA Helps Identify ‘Molly’ (Frisch/Lancour) Morelli’s Father,” National Genealogical Society Quarterly 106 (December 2018): 293-306.