Workshop: How to get the most out of your DNA test results

Mayo Genealogy Group

2 p.m. Saturday 8 September 2018

Mayo Education Centre, Castlebar

by Paddy Waldron

WWW version:

YouTube version:



DNA testing is not a substitute for genealogical research; rather the two approaches help to corroborate each other.
Genealogists compile family histories by matching up three categories of information:
  1. the oral traditions passed down through the generations;
  2. the archival sources used by traditional genealogists; and
  3. the DNA evidence that often reconciles both, but sometimes refutes either or both (NPE).
The boundaries between oral and archival can be blurred:
For example, the 1991 Cosgrove-Corry family history by Fr. Peter Ryan et al. suggests that his GGgrandfather Michael Ryan of Gower (TAB 1825) may have been brother of Dan'l Ryan of Glasscloone (TAB 1826). Can DNA add anything to the oral history and the TAB evidence?

Stanford historian Richard White wrote in his family history Remembering Ahanagran: Storytelling in a Family's Past (Cork University Press, 1999, p. 4):
I once thought of my mother's stories as history. I thought memory was history. Then I became a historian, and after many years I have come to realize that only careless historians confuse memory and history. History is the enemy of memory. The two stalk each other across the fields of the past, claiming the same terrain. History forges weapons from what memory has forgotten or suppressed. Few non-historians realize how many scraps a life leaves. These scraps do not necessarily form a story in and of themselves, but they are always calling stories into doubt, always challenging memories, always trailing off into forgotten places.
The emergence of genetic genealogy has turned this two-way struggle between memory and history into a three-way battle.

You can combine your DNA and your known ancestry to help your long-lost cousins to find you and to find your and their long-forgotten ancestors.

What is DNA?

Where does our DNA come from?

male offspring female offspring
sperm Y chromosome X chromosome
22 paternal autosomes
egg X chromosome
22 maternal autosomes

Inheritance paths

Y chromosome
Only males have a Y chromosome.
The Y chromosome comes down the patrilineal line - from father, father's father, father's father's father, etc.
This is the same inheritance path as followed by surnames, grants of arms, peerages, etc.
X chromosome
Males have one X chromosome, females have two.
X DNA may come through any ancestral line that does not contain two consecutive males.
Blaine Bettinger's nice colour-coded blank fan-style pedigree charts show the ancestors from whom men and women can potentially inherit X-DNA.
Exactly 50% of autosomal DNA comes from the father and exactly 50% comes from the mother.
Due to recombination, on average 25% comes from each grandparent, on average 12.5% comes from each greatgrandparent, and so on.
Siblings each inherit 50% of their parents' autosomal DNA, but not the same 50% (except for identical twins).
Similarly, siblings each inherit 50% of their mother's X DNA, but not the same 50% (except for identical twins).
Sisters each inherit 100% of their father's X DNA.
Everyone has mitochondrial DNA.
Mitochondrial DNA comes down the matrilineal line - from mother, mother's mother, mother's mother's mother, etc.
The surname typically changes with every generation in this line.

The Autosomal DNA and Genetic Genealogy Websites

You must link your DNA match list and your pedigree chart and share them on the four major autosomal DNA comparison websites:

Commercial and Marketing Priorities

The DNA company to which you pay your money and send your sample has a number of priorities, in this order:
  1. separating you from your money;
  2. assigning ethnicity labels to percentages of your DNA; and
  3. sending you elsewhere for help in finding cousins and ancestors.

Before you get your DNA results ...

Identity v. Anonymity

The Basic Rules

Reveal your birth surname:
Most people inherit DNA with their birth surname, so identify yourself as a minimum by your birth surname with an initial or a title, e.g., P Waldron or Mr Waldron or Miss Durkan.
Reveal the gender of the person who provided the DNA sample:
Valuable additional inferences can potentially be drawn once it is known whether two X chromosomes (female) or one X chromosome and one Y chromosome (male) are potentially available for comparison.
Women do not have Y-DNA and are encouraged to recruit their male relatives to provide Y-DNA, but they must NOT attach a female name to a male DNA sample, as this causes untold confusion.
Be especially careful not to inadvertently link a male's Y-DNA results with a female's autosomal DNA results at where error-checking does not look for this.
Avoid providing irrelevant information:
Your first name, married surname, adopted surname or marital status reveal nothing about your DNA, so you may keep these private if you wish.
Avoid pseudonyms:
They reduce the chances that your matches will bother to look at your family tree, contact you or share the information about your ancestry that they have and that you do not have.
Be consistent and avoid unnecessary confusion:
A real example (further anonymised):
Keep all your DNA-related correspondence in a single searchable e-mail archive
Use AncestryDNA or MyHeritage or Facebook messages only to exchange e-mail addresses.

After you get your DNA results ...

How can we use DNA in genealogy?

Using the tools

'One-to-one' compare
  • *Molly above and her daughter are at AncestryDNA and eventually copied their results to GEDmatch.
  • The daughter is an AncestryDNA match to her mother's probable 4th cousin, but the mother is not.
  • This is one of the few cases where one should reduce the "SNP count minimum" and "Minimum segment cM" size.
  • What does GEDmatch tell us?
  • Triangulation
    Are your parents related?
    Autosomal matrix

    Conclusion: Why you should submit your DNA

    Further reading