Genetic Genealogy for Beginners

Introduction to Irish Family History

6 p.m. Monday 29 January 2024

Thomas Davis Theatre, Arts Building, Trinity College Dublin

by Paddy Waldron

WWW version:





Your sources

Your conclusions


Communities (and more caveats)


Components of DNA

male offspring female offspring
sperm Y chromosome X chromosome
paternal autosomal chromosomes 1-22
egg X chromosome
maternal autosomal chromosomes 1-22

Inheritance paths

See pedigree chart.
Sex chromosomes
Everyone has two sex chromosomes: males XY, females XX.
Y chromosome
Only males have a Y chromosome.
Y-DNA 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., so Y-DNA is used for surname projects.
X chromosome
Males have one X chromosome, females have two.
X-DNA may come through any ancestral path that does not contain two consecutive males.
Blaine Bettinger's colour-coded blank fan-style pedigree charts show the ancestors from whom men and women can potentially inherit X-DNA.
Short for autosomal chromosomes
Exactly 50% of autosomal DNA (atDNA) 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.
In extreme cases, an individual can inherit up to 35% from one paternal grandparent and, hence, as little as 15% from the other paternal grandparent.
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.
Hence, autosomal DNA is used to produce estimated ethnicity percentages.
Everyone has mitochondrial DNA (mtDNA).
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 following table summarises these critical distinctions:
DNA component Inheritance path Inherited by
Y chromosome From father only (and only if male) males only
autosomal chromosomes Equally from both parents everyone
X chromosome(s) Unequally from both parents males x1, females x2
mitochondrial DNA From mother only everyone


Mutations are the first type of random variation in the inheritance process (of all four components of DNA), and are like transcription errors.
Special types of mutations:

Y-DNA Mutations

Autosomal DNA Mutations


Recombination is the second type of random variation in the inheritance process (of autosomal DNA and maternal X-DNA only) and is how, e.g., the father's paternal and maternal autosomes cross over to produce the child's paternal autosomes.

Using the autosomal DNA websites

It you want to identify your long-lost cousins, to help them to find you, and to identify your and their long-forgotten ancestors, then you must link your DNA data and your known ancestry in the form of a pedigree chart and share them on all the major autosomal DNA comparison websites ("fish in all the gene pools").
To add your information to the online DNA databases:

Some basic common principles:

The basic rules for successful use of the DNA websites include the following:
Reveal the DNA subject's 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.
Also take care not to link a male DNA sample to a female's pedigree chart (or vice versa), e.g. your spouse's pedigree chart (a common error).
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.
Keep all your DNA-related correspondence in a single searchable e-mail archive
Use the internal messaging system and AncestryDNA/MyHeritage/23andMe/LivingDNA or Facebook messages only to exchange e-mail addresses.

A case study: the Lynches of Moveen West

Episode 1

Episode 2

Further reading