How to Use DNA Testing to Trace Your Family Tree

Technician Doing an Experiment on DNA Sequencing in a Laboratory
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DNA, or deoxyribonucleic acid, is a macromolecule that contains a wealth of genetic information and can be used to better understand relationships between individuals. As DNA is passed down from one generation to the next, some parts remain almost unchanged, while other parts change significantly. This creates an unbreakable link between generations and it can be of great help in reconstructing our family histories.

In recent years, DNA has become a popular tool for determining ancestry and predicting health and genetic traits thanks to the increasing availability of DNA-based genetic testing. While it can't provide you with your entire family tree or tell you who your ancestors are, DNA testing can:

  • Determine if two people are related
  • Determine if two people descend from the same ancestor
  • Find out if you are related to others with the same surname
  • Prove or disprove your family tree research
  • Provide clues about your ethnic origin

DNA tests have been around for many years, but it is only recently that it has become affordable for a mass market. Ordering a home DNA test kit can cost less than $100 and usually consist of a cheek swab or a spit collection tube that allows you to easily collect a sample of cells from the inside of your mouth. A month or two after mailing in your sample, you'll receive the results—a series of numbers that represent key chemical "markers" within your DNA.

These numbers can then be compared to results from other individuals to help you determine your ancestry.

There are three basic types of DNA tests available for genealogical testing, each serving a different purpose: 

Autosomal DNA (atDNA) 

(All lines, available for both men and women)

Available for both men and women, this test surveys 700,000+ markers on all 23 chromosomes to look for connections along all of your family lines (maternal and paternal).

The test results provide some information about your ethnic mix (the percentage of your ancestry that comes from Central Europe, Africa, Asia, etc.), and helps to identify cousins (1st, 2nd, 3rd, etc.) on any of your ancestral lines. Autosomal DNA only survives recombination (the passing down of DNA from your various ancestors) for an average of 5–7 generations, so this test is most useful for connecting with genetic cousins and connecting back to more recent generations of your family tree.

mtDNA Tests 

(Direct maternal line, available for both men and women)

Mitochondrial DNA (mtDNA) is contained in the cytoplasm of the cell, rather than the nucleus. This type of DNA is passed by a mother to both male and female offspring without any mixing, so your mtDNA is the same as your mother's mtDNA, which is the same as her mother's mtDNA. mtDNA changes very slowly, so if  two people have an exact match in their mtDNA, then there is a very good chance they share a common maternal ancestor, but it is hard to determine if this is a recent ancestor or one who lived hundreds of years ago. It is important to keep in mind with this test that a male's mtDNA comes only from his mother and is not passed on to his offspring.

Example: The DNA tests that identified the bodies of the Romanovs, the Russian imperial family, utilized mtDNA from a sample provided by Prince Philip, who shares the same maternal line from Queen Victoria.

Y-DNA Tests 

(Direct paternal line, available for males only) 

The Y chromosome in the nuclear DNA can also be used to establish family ties. The Y chromosomal DNA test (usually referred to as Y DNA or Y-Line DNA) is only available for males, since the Y chromosome is only passed down the male line from father to son. Tiny chemical markers on the Y chromosome create a distinctive pattern, known as a haplotype, that distinguishes one male lineage from another. Shared markers can indicate relatedness between two men, though not the exact degree of the relationship. Y chromosome testing is most often used by individuals with the same last name to learn if they share a common ancestor.

Example: The DNA tests supporting the probability that Thomas Jefferson fathered the last child of Sally Hemmings were based on Y-chromosome DNA samples from male descendants of Thomas Jefferson's paternal uncle, since there were no surviving male descendants from Jefferson's marriage.

Markers on both mtDNA and Y chromosome tests can also be used to determine an individual's haplogroup, a grouping of individuals with the same genetic characteristics. This test may provide you with interesting information about the deep ancestral lineage of your paternal and/or maternal lines.

Since Y-chromosome DNA is found only within the all-male patrilineal line and mtDNA only provides matches to the all-female matrilineal line, DNA testing is only applicable to lines going back through two of our eight great-grandparents - our father's paternal grandfather and our mother's maternal grandmother. If you want to use DNA to determine ancestry through any of your other six great-grandparents you will need to convince an aunt, uncle, or cousin who descends directly from that ancestor through an all-male or all-female line to provide a DNA sample.

Additionally, since women don't carry the Y-chromosome, their paternal male line can only be traced through the DNA of a father or brother.

What You Can and Can't Learn From DNA Testing

DNA tests can be used by genealogists to:

  1. Link specific individuals (e.g. test to see whether you and a person you think may be a cousin descend from a common ancestor)
  2. Prove or disprove the ancestry of people sharing the same last name (e.g. test to see if males carrying the CRISP surname are related to each other)
  3. Map the genetic orgins of large population groups (e.g. test to see whether you have European or African American ancestry)


If you're interested in using DNA testing to learn about your ancestry you should start by narrowing down a question you are trying to answer and then select the people to test based on the question. For example, you may wish to know if the Tennessee CRISP families are related to the North Carolina CRISP families.

To answer this question with DNA testing, you would then need to select several male CRISP descendants from each of the lines and compare the results of their DNA tests. A match would prove that the two lines descend from a common ancestor, though would not be able to determine which ancestor. The common ancestor could be their father, or it could be a male from over a thousand years ago.

This common ancestor can be further narrowed down by testing additional people and/or additional markers.

An individual's DNA test provides little information on its own. It is not possible to take these numbers, plug them into a formula, and find out who your ancestors are. The marker numbers provided in your DNA test results only begin to take on genealogical significance when you compare your results with other people and population studies. If you don't have a group of potential relatives interested in pursuing DNA testing with you, your only real option is to input your DNA test results into the many DNA databases starting to spring up online, in the hopes of finding a match with someone who has already been tested. Many DNA testing companies will also let you know if your DNA markers are a match with other results in their database, provided that both you and the other individual have given written permission to release these results.

Most Recent Common Ancestor (MRCA)

When you submit a DNA sample for testing an exact match in the results between you and another individual indicates that you share a common ancestor somewhere back in your family tree. This ancestor is referred to as your Most Recent Common Ancestor or MRCA.

The results on their own will not be able to indicate who this specific ancestor is, but may be able to help you narrow it down to within a few generations.

Understanding the Results of Your Y-Chromosome DNA Test (Y-Line)

Your DNA sample will be tested at a number of different data points called loci or markers and analyzed for the number of repeats at each of those locations. These repeats are known as STRs (Short Tandem Repeats). These special markers are given names like DYS391 or DYS455. Each of the numbers that you get back in your Y-chromosome test result refer to the number of times a pattern is repeated at one of those markers.

The number of repeats is referred to by geneticists as the alleles of a marker.

Adding additional markers increases the precision of DNA test results, providing a greater degree of probability that a MRCA (most recent common ancestor) can be identified within a lower number of generations. For example, if two individuals match exactly at all loci in a 12 marker test, there is a 50% probability of a MRCA within the last 14 generations. If they exactly match at all loci in a 21 marker test, there is a 50% probability of a MRCA within the last 8 generations. There is a fairly dramatic improvement in going from 12 to 21 or 25 markers but, after that point, the precision starts to level off making the expense of testing additional markers less useful. Some companies offer more precise tests such as 37 markers or even 67 markers.

Understanding the Results of Your Mitochondrial DNA Test (mtDNA)

Your mtDNA will be tested on a sequence of two separate regions on your mtDNA inherited from your mother.

The first region is called Hyper-Variable Region 1 (HVR-1 or HVS-I) and sequences 470 nucleotides (positions 16100 through 16569). The second region is called Hyper-Variable Region 2 (HVR-2 or HVS-II) and sequences 290 nucleotides (positions 1 though 290). This DNA sequence is then compared to a reference sequence, the Cambridge Reference Sequence, and any differences are reported.

The two most interesting uses of mtDNA sequences are comparing your results with others and determining your haplogroup. An exact match between two individuals indicates that they share a common ancestor, but because mtDNA mutates extremely slowly this common ancestor could have lived thousands of years ago. Matches which are similar are further classified into broad groups, known as haplogroups. A mtDNA test will provide you with information about your specific haplogroup which may provide information on distant family origins and ethnic backgrounds.

Organizing a DNA Surname Study

Organizing and managing a DNA surname study is very much a matter of personal preference. There are, however, several basic goals which need to be met:

  1. Create a Working Hypothesis: A DNA Surname Study is not likely to provide any meaningful results unless you first determine what you are trying to accomplish for your family surname. Your goal can be very broad (how are all the CRISP families in the world related) or very specific (do the CRISP families of eastern NC all descend from William CRISP).

     

  1. Choose a Testing Center: Once you've determined your goal you should have a better idea of what type of DNA testing services you will require. Several DNA Laboratories, such as Family Tree DNA or Relative Genetics, will also assist you with setting up and organizing your surname study. 

     

  2. Recruit Participants: You can reduce the cost per test by assembling a large group to participate at one time. If you are already working together with a group of people on a particular surname then you may find it relatively easy to recruit participants from the group for a DNA surname study. If you have not been in touch with other researchers of your surname, however, you will need to track down several established lineages for your surname and obtain participants from each of these lines. You may wish to turn to surname mailing lists and family organizations to promote your DNA surname study. Creating a website with information about your DNA surname study is also an excellent method for attracting participants.

     

  1. Manage the Project: Managing a DNA surname study is a big job. The key to success is in organizing the project in an efficient manner and keeping participants informed of progress and results. Creating and maintaining a Web site or mailing list specifically for project participants can be of great assistance. As mentioned above, some DNA testing labs will also provide assistance with organizing and managing your DNA surname project. It should go without saying, but it is also important to honor any privacy restrictions made by your participants.

    The best way to figure out what works is to look at examples of other DNA Surname Studies. Here are several to get you started:

    It is vitally important to keep in mind that DNA testing for the purposes of proving ancestry is not a substitute for traditional family history research. Instead, it is an exciting tool to be used in conjunction with family history research to aid in proving or disproving suspected family relationships.

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    Powell, Kimberly. "How to Use DNA Testing to Trace Your Family Tree." ThoughtCo, Oct. 16, 2017, thoughtco.com/dna-family-trees-1420576. Powell, Kimberly. (2017, October 16). How to Use DNA Testing to Trace Your Family Tree. Retrieved from https://www.thoughtco.com/dna-family-trees-1420576 Powell, Kimberly. "How to Use DNA Testing to Trace Your Family Tree." ThoughtCo. https://www.thoughtco.com/dna-family-trees-1420576 (accessed October 22, 2017).