Using DNA to fill gaps in family histories

Irish Family History Society

3:15 p.m. Saturday 10 March 2018

Dublin City Library & Archive, 138/144 Pearse street, Dublin 2

by Paddy Waldron

WWW version:

YouTube version:



Family history flows down through the generations via three channels:

  1. Most genealogists start by writing down the oral traditions passed down through the generations to older family members.
  2. Archival sources are then traditionally used by genealogists to fill some of the gaps in the oral family history.
  3. DNA evidence now fills many of the remaining gaps left by the traditional sources.
Talk to your known relatives and correspond with your closest DNA matches. Sometimes the gaps left by one individual's sources can be filled by those of a known or unknown relative.
The boundaries between oral and archival can be blurred:
All three types of sources must be consulted. None can be ignored.

The genealogist has to be judge and jury.

An example:

NPEs can be revealed by oral, archival or genetic sources, most often by the latter.

This talk will outline the steps necessary to 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).
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 major autosomal DNA comparison websites: only got its matching algorithm working properly in late 2017, so that Jim Palmer was not sure whether the match which helped to identify his birth mother was genuine. can not currently be recommended for genealogy for several reasons:

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 (percentages based on centiMorgans, SNPs, base pairs or what?); 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 find you or 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 Facebook messages only to exchange e-mail addresses.

After you get your DNA results ...

How can we use DNA in genealogy?

Example I: How I found my O'Dea GGGGGgrandmother

Now I am co-administrator of the O'Dea/O'Day/Dee (DNA) Project (Y-DNA) and we also have an O'Deas of Newtown project (autosomal DNA).

Example II: How I helped my 4th cousin to find multiple generations of Clancy ancestors

Now I am co-administrator of the Clancy Surname (DNA) Project.

Conclusion: Why you should submit your DNA

Further reading