DNA and Genealogy

Tom Dreyer


DNA has become an important tool in genealogical research, and as more people take the test, the number of matches increases, making it even more useful. Unfortunately, when many people receive the results of their test, they are unsure what they mean or what to do with them. The goal of this subject guide is to explain the tests and to show what tools are available for research and how to use them to identify ancestors and previously unknown relatives. Words in blue are linked to definitions in the glossary at the end of this guide.

The Biology

Autosomal DNA

We are each endowed with twenty-three pairs of chromosomes, twenty-two pairs of non-sex chromosomes, and one pair of sex chromosomes (XX [female] or XY [male]).

Mitochondrial DNA

Separate from the chromosomes, each of us also receives mitochondrial DNA from our biological mothers. This DNA is passed down almost unchanged from the mother to all of her children, but only the daughters will pass it down virtually unchanged to her children.

The Tests

Choosing a DNA Test for Family History Research

Live broadcast: June 15, 2017
Presented by: Christopher C. Child, Senior Genealogist
Level: Beginner - Intermediate Running Time: 1:03:39
Description: Advances in DNA research over the last decades have had huge implications for the field of genealogy. In this webinar Christopher C. Child, editor of the Genetics & Genealogy column in American Ancestors magazine discusses what types of genetic tests are available to family historians, what genealogical problems they can—and can’t—assist you with, and ultimately how to choose the right test(s) to further your family history research.

Autosomal DNA (or atDNA)

The autosomal test is the most popular DNA test taken. It examines the 22 non-sex chromosome pairs and the X chromosome on chromosome 23. It helps to identify distant living relatives and, through them, ancestors you have in common.


The Y-DNA test is taken by males only, as only males have a Y chromosome in chromosome 23. Because females lack this chromosome they will often ask a paternal male relative to take the ‘Y’ test; often a father, paternal cousin, paternal uncle or nephew. The Y-DNA test can be useful for identifying ancestors in a direct male line.


Mitochondrial DNA, which is not part of the chromosomes, is passed down relatively unchanged from a mother to all her children. Only her daughters (not her sons) can continue to pass the mitochondrial down to future generations.

Your Test Results

Autosomal DNA

All the test companies provide you with matches to other individuals you share DNA with. The more DNA you share, the closer the relationship. You will also receive an estimate from the testing company as to the closeness of the relationship. This can range from very close (parent/child) to distant (5th cousin or greater). The chart above shows two very close matches (children) and more distant matches.

You will also receive an estimate of the origins of your distant ancestors. These origins are from thousands of years ago. The chart above shows a predominance of distant ancestors from Western Europe.


The companies test certain segments of Y-DNA called markers. Currently testing companies offer tests showing 700, 111, 67, 37, 25, and 12 markers on the Y chromosome. The chart above illustrates a 37-marker test. This test traces your paternal line from father to father and can pinpoint a male ancestor within a certain number of generations. The Big Y-700 marker test is the most expensive but makes it possible to identify an ancestor living within the last few generations. A 12-marker test (even an exact match) may not be more precise than a thousand years ago or more. The ideal test for genealogical purposes is the 37 or the 111-marker test.

From these markers the testing companies can estimate which paternal haplogroup you belong to. This indicator can tell you where your ancestors settled after migrating out of Africa. The map above shows the path of migration for paternal haplogroups. In this example, it can be seen that the R1 haplogroup of this test taker migrated to Western Europe.


Before a full mitochondrial sequence test was available, the test examined only two regions of the mitochondrial DNA named HVR1 and HVR2. Recently, however, the test now includes all 16,569 base pairs, which results in matches which may be closely related to you. Shown here is a chart which compares the DNA of a test taker to the RSRS (Reconstructed Sapiens Reference Sequence).

As with the Y-DNA test, the testing companies will tell you what genetic distance you are from your matches. The lower the number, the closer your relationship.

Similar to the Y-DNA test, elements of the mitochondrial DNA determine your haplogroup and from this the migration path of your ancient female ancestors after leaving Africa. The chart above shows the ‘T’ haplogroup migrating to Western Europe.

Leading Test Companies and Prices*

African AncestryN/A$299.00$299.00
Ancestry Inc.$99.00N/AN/A
Family Tree DNA$79.00Y-37: $119.00
Y-111: $249.00
Big Y-700: $449.00
Living DNA$99.00**N/AN/A
My Heritage$99.00N/AN/A
Oxford Ancestors (British) 26N/A£199.00N/A

*does not include frequent discounts
**includes limited Y-DNA and mtDNA testing

Tools Offered (Autosomal)

In addition to matches, origins, and estimated relationships, each testing company provides tools to assist in genealogical research. Below is a chart of offerings:

AncestryFamilyTree DNA23&Me
Genetic CommunitiesYNN
Family TreesYYN
One-to-Many MatchesYYY
In-Common-with MatchesYYY
Health InformationNNY**

*DNA Circles

Tools Offered (Y-DNA)

Group Surname Projects – A website dedicated to accumulating the Y-DNA for all individuals with the same surname. For example, the Y-DNA for all male individuals who share a common surname is collected and sorted by identical DNA markers into groups with the oldest known male ancestor included. Those who match closely are likely related through the same ancestral line. The above sample is a part of the Smith Group Surname Project page. You can see that all individuals in the second group only took the 12-marker test. Limited space prevented showing the results of other groups and those who took a 37-marker test or greater.

Third-Party Websites

All the above testing companies allow you to download your DNA information and upload it to third-party websites. While these sites do not generally offer testing, they often provide more tools than are available from the testing companies themselves. Some popular examples are:

American AncesTREESChromosome Browser
One-to-Many Match
Gedmatch.comOne-to-Many Match
One-to-One Compare (Chromosome Browser)
3D Chromosome Browser Phasing
People who match one or both of two kits
DNAGedcom.comG Works – analyzes GEDCOMs and family-tree matches posted by your autosomal DNA matches on Ancestry or Family Tree DNA
JWorks and KWorks – sorts your data by DNA segments into overlapping DNA and assigns ICW status within sets
Genome Mate ProOne-to-Many Matches
Organizational Tools

Using DNA Tools in American AncesTREES

Live broadcast: December 19, 2019
Presented by: Tom Dreyer
Level: All Running Time: 1:07:33
Description: American AncesTREES is an online genealogy tree experience on AmericanAncestors.org that offers several unique features—including cutting-edge DNA tools. Join Genealogist Tom Dreyer to learn more about these DNA solutions, how they can be used to interpret your DNA results, and how the information can be incorporated into your family tree!

More About DNA

Autosomal DNA

DNA from the twenty-two pairs of non-sex chromosomes is referred to as autosomal DNA. Autosomal DNA is inherited from both parents; approximately 50 percent from the father and 50 percent from the mother. Thus, the remaining DNA from each parent is lost to the child. A sibling also inherits 50 percent from each parent but not necessarily the same DNA. He/she may inherit some of the DNA that was lost to his or her sibling and may also fail to inherit some DNA that the other sibling did receive. Since this recombination occurs in each generation, the amount of DNA received from a particular individual declines by about 50 percent with each succeeding generation. Eventually, it is either completely lost or becomes such a small amount that it is impossible to identify as coming from that individual. For example, a child receives about 1 ½ percent of DNA from each of his/her 4th great-grandparent. Some people will receive a bit more, and some may receive none at all.

Due to the size of the chromosomes, fully testing all your autosomal DNA is at this time too costly. Because of this, the testing companies test hundreds of thousands of representative pieces of DNA ( SNPs) on the 22 non-sex chromosomes plus SNPs from the X DNA (on chromosome 23).

Chromosome 23 (The Sex Chromosome)

The 23rd chromosome is the sex chromosome pair. In males, the pair is XY and in females XX.

The Y chromosome, which is present only in the male, is passed down from father to son only. An SNP test is available to determine a more definitive Y haplogroup than the STR test provides.

The X chromosome, of which males have one and females have two, can be passed down from the mother to all her children but can be passed down from the father only to his daughters. Due to this unique inheritance characteristic and the fact that testing companies currently test fewer SNPs on the X chromosome, only large segments should be analyzed when evaluating matches. The X chromosome testing results are included in the autosomal test, and as noted above the Y chromosome is tested separately with various numbers of markers (37, 67, 111).

What Are SNPs, Markers, and STRs?

An SNP (or Single Nucleotide Polymorphism) is simply the bit of DNA found at a specific location on a chromosome. There are four nucleotides that make up our DNA: adenine, cytosine, guanine, and thymine. They are represented by the letters A, C, G, and T.

Markers are blocks of SNPs. There are 111 blocks or markers in the 111 Y-DNA test. Each marker is assigned a DNA Y segment (DYS) designation. The above illustration shows 37 different markers.

STR is an acronym for Short Tandem Repeat. Strings of DNA often repeat within a marker. The number of repeats which occurs in each marker is assigned as the value for that marker or Y DNA Segment (DYS). In the example shown, a value of 15 in marker DYS393 indicates 15 repeats at that location. If you and your Y-DNA match both have an identical value in all your markers you are said to be a genetic distance of ‘0’ from each other. This may indicate a close relationship with your match. The more markers which match, the closer the relationship. However, a ‘0’ genetic distance on a 12-marker test may indicate a relationship several thousand years ago; whereas a ‘0’ genetic distance on a 111 Y-DNA test would signify a much closer relationship of a few generations or less.

How to Use Some Popular Tools

Autosomal Matches – CentiMorgans (cMs) – a term which you will undoubtedly see regarding the autosomal test – are expressed as cMs. This is a unit of measurement which is used to show how much DNA you share with a person who matches you. The more cMs you share, the closer your relationship is to that person.

Once your matches have been presented to you by the testing companies, you will likely want to see if those matches have a family tree online and/or contact them to find how you are related and who your most common ancestor is. You will have the most success with those matches who share the most cMs (centiMorgans) with you. You and your match should share common surnames in your family tree and the places in which ancestors lived. The more complete your and your match’s family tree is, the more likely you are to be successful in locating your most recent common ancestor.

DNA ThruLines – DNA ThruLines searches your matches’ trees to locate a most recent common ancestor recent common ancestor. This tool is useful for identifying both living relatives and common ancestors up to 6 generations in the past. By clicking on your ancestor in this chart, you can discover other DNA matches who share that common ancestor. This illustrates how having a public family tree can help identify common ancestors.

DNA Circles – A DNA Circle is a list of individuals who as a group share a most recent common ancestor in their family tree and autosomal DNA. This tool is useful for identifying both living relatives and common ancestors up to 6 generations in the past. This chart lists DNA matches who also have Christian Hesse as an ancestor in their family tree. This illustrates how having a public family tree can help identify common ancestors.

Genetic Communities – These are created by combining genetic information with geographic data from family trees. The result is a more specific estimate of the geographic origins of your family and its migration pattern. Generally, test takers will be assigned from one to three genetic communities. You may have an individual with whom you do not share DNA included in your Genetic Community. This can occur because they share a common ancestor in their family tree and they also share DNA with other testers who also have the same ancestor in their tree.

One-to-Many Tool – When you are notified that you are a genetic match with another person, most one-to-many tools will present a chart with all your matches, usually in order by the total number of centiMorgans (cMs) which you share with each match. The size of the largest block of centiMorgans is often shown as well. In addition, a name and/or an email address is given for each match so that you can contact them to compare family trees and to determine who your common ancestor may be.

One-to-One Compare (Chromosome Browser) – Once you have identified someone with whom you share DNA, a chromosome browser will enable you to identify the exact amount and locations on specific chromosomes you have in common. The above chart shows that the person who took the DNA test and the matching person share 52.9 centiMorgans on chromosome 4. The number of cMs in a segment, the number of matching segments and the total number of centiMorgans you share are all considered when evaluating a relationship. The figure below is a chart which shows approximately how many cMs each of your matches shares with you.

In Common With – When you have found a match using a chromosome browser and identified your most common ancestor, the In Common With (ICW) tool can identify other matches whose DNA you both share with other testers. This can lead to finding relatives from the same family line. The image above reveals all the people who match two known relatives who share DNA.


Tree Triangulation – Identify close DNA matches and compare your family tree to your match’s tree to locate a common ancestor. You will want to pay close attention to common surnames and geographic locations.

Triangulation – Using a chromosome browser, identify segments on specific chromosomes which you and a match share with others; then examine a matching tree which may lead to a common ancestor. This diagram shows all matches who share the same segments of DNA on chromosome 1 with two known relatives. Discovering that this DNA segment came down to you from a certain ancestor may help in finding other family members in the same branch of your family tree who also share this DNA segment.


Autosomal DNA (atDNA) - Autosomal DNA is DNA from one of our chromosomes located in the cell nucleus. It generally excludes the sex chromosomes. Humans have 22 pairs of autosomal chromosomes and a pair of sex chromosomes.

centiMorgan - A centiMorgan (cM) is a measurement of how likely a segment of DNA is to recombine from one generation to the next. A single centiMorgan is considered equivalent to a 1% (1/100) chance that a segment of DNA will crossover or recombine within one generation.

Chromosome - A chromosome is a structure found in the nucleus of a cell that contains genetic material. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.

DNA segment - A DNA segment is any continuous run or length of DNA. It is described by the place where it starts and the place where it stops.

GEDCOM file (Genealogical Data Communication) - A Genealogical Data Communication (GEDCOM) file is a special file format that was developed to provide a standard for encoding genealogical data. It is not used by most family tree software packages but most can import and export to GEDCOM format. Because of this, it is today used by many genealogists to exchange pedigree data files.

Genetic Distance - There are two meanings for Genetic Distance:

  1. Genetic Distance is the number of differences, or mutations, between two sets of results. A genetic distance of zero means there are no differences in the results being compared against one another, i.e., an exact match. This is the meaning when comparing Y-chromosome DNA or mitochondrial DNA.
  2. For autosomal DNA comparisons, genetic distance may refer to the size of a DNA segment. The genetic distance is then the length of the segment in centiMorgans.

Genetic Genealogy - Genetic genealogy is the use of your DNA to solve genealogy puzzles.

Genome - A genome is the entire complement of an organism’s genetic material. This may refer to the DNA of a gamete, organelles (mitochondria and chloroplasts), organism, or species.

The human nuclear genome is composed of 46 chromosomes (23 pairs). They contain a total of 3 billion base pairs.

The human mitochondrial genome is composed of a single circular DNA sequence that contains 16569 base pairs.

Haplogroup - A haplogroup is a major branch on either the maternal or paternal tree of humankind. Haplogroups are associated with early human migrations. Today these can be associated with a geographic region or regions.

Haplotype - A haplotype is the set of values for a set of DNA values. For example, the results of the Y-DNA12 test for one person is their haplotype.

Two individuals that match exactly on all markers have the same haplotype.

HVR1 (Hyper-variable Control Region 1) - One of two segments of mitochondrial DNA which changes relatively quickly.

HVR2 (Hyper-variable Control Region 2) - A second segment of mitochondrial DNA which changes relatively quickly.

Identical by Descent (IBD) - IBD stands for identical by descent. This means the DNA matches because it comes from a common ancestor. IBD can refer to a single mutation or to a segment of DNA. If a mutation or segment of DNA is IBD among a group of people, it comes from a common ancestor.

Identical by State (IBS) - IBS stands for identical by state, meaning the DNA matches by coincidence. When two individuals share numerous individual results without being related, those results are IBS.IBS stands for identical by state, meaning the DNA matches by coincidence. When two individuals share numerous individual results without being related, those results are IBS.

In Common With (ICW) - A tool used to identify DNA matches who share DNA with yourself and another match.

Longest Block - The Longest Block in the autosomal Family Finder test refers to the longest continuous segment of autosomal DNA that is shared between two individuals.

Marker - A marker is a physical location (locus) on the chromosome. The term is often used colloquially in genetic genealogy to refer to a short tandem repeat (STR). For example, “The Y-DNA67 test is a panel of 67 markers.”

Mitochondrial DNA - The genetic material found in mitochondria. It is passed down from females to both sons and daughters, but sons do not pass down their mother’s mtDNA to their children.

MRCA - The most recent common ancestor which you and a DNA match share.

Phasing - A tool used to separate the autosomal DNA received from the father and autosomal DNA received from the mother.

Recombination - Recombination is the mixing of the DNA on each chromosome that you receive from your mother and father. Different chromosomes and different parts of each chromosome are more or less likely to recombine in a single generation.

RSRS (Reconstructed Sapiens Reference Sequence) - A recent effort by scientists to represent one ancestral genome for the entire human population.

Sex Chromosome - The X or Y chromosome. Normally males have one X and one Y and females have two Xs.

Short Tandem Repeat (STR) - A short DNA motif (pattern) repeated in tandem. ATGC repeated eleven times would give the marker a value or allele of 11.

SNP (Single Nucleotide Polymorphism) - A single nucleotide of DNA of which there are four types; (A) adenine, (C) cytosine, (G) guanine, and (T) thymine

Triangulation - Segment Triangulation: Using DNA segments you share with a known relative to identify common ancestors of other matches who share DNA on the same segments. Tree Triangulation: Comparing family trees of DNA matches to locate a common ancestor.

X Chromosome - One of the two sex chromosomes, X and Y. The X chromosome is passed down from mother to child. X is the sex chromosome that is present in both sexes, singly in males and doubly in females.

Y Chromosome - One of the two sex chromosomes, X and Y. The Y chromosome passes down from father to son. Females do not receive it. As the Y chromosome is passed on through the paternal line, it is valuable for surname based genealogy studies.

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