Handout on Genetic Genealogy
Getting Started in Genetic Genealogy
Genetic genealogy is the process of using DNA tests to determine how people are related through
shared DNA (by “blood”). To better understand this rapidly evolving field, the International Society of
Genetic Genealogy (http://www.isogg.org/) has a useful guide and glossary for “newbies”. Also see
“Definitions of the terms used in genetic genealogy” at the FamilyTreeDNA website for more
definitions: https://www.familytreedna.com/faq/answers/default.aspx?faqid=21
There are four main types of DNA used for genetic genealogy. Autosomal DNA is the most useful for
general genealogy in recent generations, although all types may help to answer particular questions.
Y chromosome is passed only from fathers to sons, so it traces only a single line of descent (patrilineal).
It is very useful for testing even distant relationships through that line.
Mitochondrial (mtDNA): passed only from mothers to children—single line of descent only
(matrilineal). It is useful for testing even distant relationships through that line, but challenging to use
due to surname changes that generally make it more difficult to trace female lines.
Autosomal (auDNA or atDNA): all chromosomes except the sex chromosomes—shared and
recombined from parents, it represents all lines but is only reliable at detecting relationships within
about 5-6 generations (see figure below).
Figure 1: Inheritance of Y DNA (tall male figures), mtDNA (tall female figures), and autosomal DNA
(other figures). Figure from Your Genetic Genealogist blog by CeCe Moore,
http://www.yourgeneticgenealogist.com/2012_04_01_archive.html
X chromosome DNA is passed from fathers to daughters or from mothers to either sons or daughters,
giving it a unique pattern of descent (see figures 2A and 2B). It can be particularly useful in narrowing
relationships identified through autosomal DNA tests. Tests for autosomal DNA include X-DNA, but
only FamilytreeDNA, 23andme, and GEDMATCH show your X-DNA results (AncestryDNA does not).
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Figure 2A: X-DNA inheritance for a male from his mother’s side (shows average percentage of X DNA
from each generation, but note that you should not necessarily “expect” the average, because the X
chromosome recombines somewhat unpredictably
(http://www.thegeneticgenealogist.com/2009/01/
12/more-x-chromosome-charts/)
Figure 2B: X-DNA Inheritance chart for females:
(http://www.thegeneticgenealogist.com/wpcontent/uploads/2008/12/1b.png) (shaded boxes
show possible sources of X DNA).
A simple rule to remember is that X DNA cannot
pass through 2 males in sequence.
For these charts, the father is on the left side and
the mother is on the right side above each cell
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below.
Basics of Autosomal DNA Testing Strategies
Each of the companies offers the basic DNA test for about $99 (note that there are often sales
throughout the year, especially around the winter holidays, Father’s and Mother’s Day, and DNA day
on April 25). Buying multiple kits can save too. This site compares the three major companies:
http://www.isogg.org/wiki/Autosomal_DNA_testing_comparison_chart
FamilyTreeDNA is particularly helpful for testing hypothesized relationships, because matches are
generally responsive, and the site has the best tools for examining your matches in detail.
AncestryDNA is particularly helpful for identifying unknown ancestors and relatives, because it has
the largest database of users with family trees and will generally yield the most matches. However, the
site lacks tools for examining your matches (see below for how to get tools!).
23andme may be helpful for finding living relatives, because they have a broad database of users
(not just genealogists!). However, many users are anonymous and lack family history information, so it
is challenging to use for genealogy. The latest version of their testing chip is no longer compatible with
GEDMATCH.com, an important site for comparing results across platforms. One perk of 23andme is
that their test does provide haplogroup information for Y DNA (for males) and mitochondrial DNA—
their results for the Y are not directly comparable to Familytreedna’s Y tests, but those results can
serve as clues that help to exclude some families.
To get the most answers to your questions, you may choose all three, and FamilyTreeDNA accepts
transfers from the other two companies for $39. The Geno 2.0 test from National Geographic provides
“deeper” (=older) ancestry than is used by genealogists, but there is a free transfer to FamilyTreeDNA
where the results may be useful, as they include some Y and mitochondrial DNA information.
Interpreting Autosomal DNA Results
The Centimorgan (cM) is a measurement of how likely a segment of DNA is to have been inherited
from a common ancestor.
>10 cM block indicates definite shared ancestry.
5-10 cM block probable shared ancestry (most companies and GEDMATCH are using a threshold of
about 7 cM to determine matches. GEDMATCH also uses 7 cM as a common threshold for matches
based upon X DNA, although it is more complicated to interpret those values because men and women
have different amounts of X DNA).
Smaller segments can indicate shared ancestry, but they may also be false positives (see post by
Roberta Estes for more: http://dna-explained.com/category/ancient-dna/).
NOTE that average amounts of shared DNA are based on assumptions that folks share a single ancestor
or pair of ancestors, rather than multiple pairs of ancestor, in recent genealogical time (the past few
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Handout on Genetic Genealogy
hundred years). The expected values and thresholds for matching will be higher in endogamous
populations—ones that have a high degree of intermarriage.
Table A: Likelihood based upon length of shared segment
Source: Tim Janzen: http://www.isogg.org/wiki/Identical_by_descent)
Length of shared
segment
>30 cM
20-30 cM
12-20 cM
6-12 cM
Likelihood you and your match share a common ancestor within 6
generations (values will be different for endogamous populations)
90%
50%
20%
5%
<6 cM
<1%
Table B: Likelihood of matching actual relatives
Sources: http://www.isogg.org/wiki/Autosomal_DNA_statistics and
http://www.familytreedna.com/faq/answers/default.aspx?faqid=17#628
Shared
DNA
Average
cM
Shared
50%
3400
25%
1700
12.50% 850
Likelihood
of
Matching
>99%
>99%
>99%
Relationship
Mother, father, siblings
Grandparents, aunts, uncles, half-siblings, double first cousins
Great-grandparents, first cousins, great-uncles, great-aunts, halfaunts/uncles, half-nephews/nieces
6.25% 425
>99%
First cousins once removed, half first cousins
3.13% 212.5
>99%
Second cousins, first cousins twice removed
1.56% 106.25
>90%
Second cousins once removed
0.78% 53.13
>90%
Third cousins, second cousins twice removed
0.39% 26.56
Third cousins once removed
0.20% 13.28
>50%
Fourth cousins
0.10% 6.64
Fourth cousins once removed
0.05% 3.32
>10%
Fifth cousins
.01%
0.83
<2%
Sixth cousins or more distant
Triangulation is the process of determining that a particular autosomal DNA segment has been
inherited from a common ancestor by identifying two or more cousins who share that segment. Note
that this does not mean that all descendants of that ancestor will have that segment, but it suggests
that the segment might be an indicator of descent from that family line.
Tools for Triangulation
GEDMATCH.com is a free, donation-supported site for comparing results across the 3 major
companies. By donating $10, you can become a “Tier 1” member that has some additional tools,
including Triangulation.
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Handout on Genetic Genealogy
The Autosomal DNA Segment Analyzer here: https://www.dnagedcom.com/adsa/index.php
triangulates your FamilyTreeDNA matches.
Genome Mate allows you to keep track of your matches across the platforms from FamilyTreeDNA,
23andme, and GEDMATCH.
Matches in common: finding all matches shared by two or more individuals. This feature is available at
FamilyTreeDNA and GEDMATCH.
Some Common Questions
Why does my known cousin not appear as a DNA match? The odds of matching depend on the degree
of the relationship (see table above)—known cousins may appear closer or more distant due to random
inheritance of DNA. As a result some cousins, even as close as 3rd cousins, may not appear in your
match lists. Also, non-paternity may also account for a lack of a relationship—the presumed relatives
actually had different fathers and/or mothers than what was expected.
Why does my sibling have different matches than I do? Because autosomal DNA is randomly inherited
from one’s parents, siblings will have somewhat different autosomal DNA. Also, note that females
inherit X DNA from their fathers and their mothers, while males inherit X-DNA only from their mothers,
so brothers and sisters have different X-DNA results. For this reason, it may be helpful to have results
from your siblings in addition to your own results.
Why do I have a relatively close match to someone, yet we cannot find our relationship? In cases
where people actually know their recent ancestors, this result may reflect having more than one shared
line of ancestry. You may look for “cousin marriages” in the trees of such individuals, which will increase
the DNA passed down by those ancestors in common.
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Handout on Genetic Genealogy
Supercousins or “Up cousins”: A person one or more generations higher than anyone alive in
your direct line, whose DNA results can help you make connections.
Two pathways for searches
Records to DNA
DNA to Records
1. Find a suspected common ancestor based
upon records or family lore
1. Find matches in common with one or more
shared DNA segments
2. Find one or more descendants to test
2. Search the trees of those matches for families
and places in common
3. Get DNA results
3. Identify likely common ancestors
4. Look if shared segments and matches in
common support the hypothesis
4. Determine if paper records support a
connection
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Handout on Genetic Genealogy
Ten strategies for using genetic genealogy to break through brick walls
1) Secure samples from the oldest generations: In your immediate family, recruit DNA samples
from the highest generation available on the line of interest. Once processed and stored with a
company like FamilyTreeDNA, DNA samples may be used for additional testing in the future.
a. Note that siblings will have somewhat different results so it can be worth getting
samples from each. In particular, males and females have different X DNA results.
b. For general searches, I recommend starting with AncestryDNA and transferring results
to FamilyTreeDNA ($39) and GEDMATCH.com (free or $10 to get triangulation).
c. If you want to validate a hypothesized relationship, going straight to FamilyTreeDNA
may be a more efficient solution because they offer more sophisticated analysis tools.
2) Build a cousin network for genetic genealogy: recruit 1st, 2nd, 3rd, 4th, and 5th degree cousins
who share descent on your lines of interest to take DNA tests. Especially seek out
“supercousins” from higher generations who carry more DNA from the ancestors of interest.
a. Note that results from cousins who have multiple lines of descent from the same
ancestors will have greater power to detect matches with those ancestors.
b. Cousins whose ancestors were half-siblings of your ancestor of interest will have a
weaker match, but their results can help you to isolate that paternal or maternal line.
c. Living cousins who would have an X DNA, Y DNA, or mitochondrial DNA connection may
be particularly valuable for validating relationships, including ones that may be too
distant for autosomal DNA to reliably trace.
3) Find the cousins to fill out your network: To identify cousins who would be helpful in your
search, you can use Wikitree and other online family trees to identify living descendants who
have tested or might be willing to test. Genealogy sites will generally yield higher responses,
but even general sites like Facebook can work, although response rates can be low.
4) Share your information so others can help you: Link your DNA results and all of your known
surnames to complete family trees so that folks can better find points of connection—let them
help you! Avoid posting partial trees (for example, your paternal or maternal family only) and
clearly identify lines that represent a known or suspected adoption.
5) Contact your matches but give them details to understand the connection: When you contact
individuals with whom you share a match, be sure to identify the type of match (i.e.,
autosomal) and the name associated with the kit. Genetic genealogists often manage results
from many individuals.
6) Systematically search your DNA results using multiple strategies:
a. Find matches in common with known relatives; make notes associating those individuals
with the shared surnames and/or locations. Note that even if you can’t trace a particular
matching individual to your family, you may be able to figure out where they connect to
your tree, and triangulate on their results to find other matches in common.
b. Search matches by surnames, particularly relatively rare ones (all three companies
permit this, although Ancestry works the best, and AncestryDNA Helper tool allows you
to search by full names. Based on paper genealogy, you may have hunches about which
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Handout on Genetic Genealogy
families are connected to yours. If you can find someone with a rare surname in that
family, you might try searching for that surname in your matches.
c. Search matches by placenames, particularly when relatively rare (Ancestry.com is best
for this kind of search)
d. Search matches by shared DNA segments (use the tools under “Triangulation”).
7) Group and sort your results: Generate lists or spreadsheets that show clusters of shared
matches by family group. You can also generate spreadsheets showing shared matches by DNA
segments on each chromosome. If someone unknown shares one of those segments, you may
be able to guess to which line they relate.
8) Use the hints (shaking leafs) at AncestryDNA to identify folks who appear to share a common
ancestor—contact those people and encourage them to upload their results to GEDMATCH so
that you can search for matches in common and compare matching DNA segments.
9) Break out the advanced tools: If you have AncestryDNA results, install Jeff Snaveley’s
“AncestryDNA helper” tool (available in the Google Chrome store for use with the Chrome web
browser) to automatically obtain lists of matches and ancestors of matches. If you have
multiple kits in your Ancestry account, you can use this to easily identify shared matches.
10) Hunt for new leads: You can use the “Ancestors of Matches” results from the AncestryDNA
Helper to find individual names (first and last name combined) that are particularly common in
your ancestry. You can sort by the “incidence” column to determine individual names that
appear multiple times. This is currently one of the best options when trying to trace a common
surname like Smith.
Final word: Genetic genealogy adds a powerful scientific tool for family historians. You will want a
skeptical frame of mind when pursuing possible leads and matches—do not discount that folks may be
related along multiple lines or that family trees may have errors.
If you have questions about any of this content, you can email me at longjonathanw@gmail.com
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