Genetic Genealogy

Irish Family History: intermediate level

7 p.m. Tuesday 13 November 2018 and Tuesday 20 November 2018

Emmet Theatre, TCD

by Paddy Waldron

WWW version:

YouTube version:

13 November - 20 November TBA

***** NB: FamilyTreeDNA kits will be available after this talk for anyone interested via the DNA Outreach IRL project *****




Components of DNA

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

Inheritance paths

See pedigree chart.
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.
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 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.
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:
Inheritance path DNA component Inherited by
From father only (and only if male) Y chromosome males only
Equally from both parents autosomal chromosomes everyone
Unequally from both parents X chromosome(s) males x1, females x2
From mother only mitochondrial DNA everyone


Mutations are the first type of random variation in the inheritance process (of all four components of DNA), and are transcription errors at single locations, e.g. a single A in the parent may be replaced by a C in the child.

Some locations mutate very frequently (every couple of generations), and can be used to identify individuals beyond reasonable doubt, e.g. in criminal cases.

Some locations mutate less frequently (only once in many generations or once in the history of mankind), and can be used to identify closely or distantly related individuals.

Special types of mutations:

Y-DNA Mutations

mtDNA 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 DNA websites

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 on several websites.
Laboratory No Yes Yes Uses FTDNA Yes Yes
DNA data uploads Yes Yes No Free to 1 Dec 2018 No No
DNA data downloads No Yes Yes Yes Incompatible No
Family tree uploads Yes Yes Yes Yes No No
Family tree downloads No No Yes Yes No No
Cousin matching Yes Yes Yes Yes Yes Promised
Customer numbers c 1 million c 1 million c 10 million c 1 million c 1 million 0
Cost excl. shipping (20 Nov 2018) Free USD49 USD59 + annual subscription EUR49 + annual subscription EUR99 GBP89
Contact matches E-mail E-mail Internal messages Internal messages Optional No
Chromosome browser Yes Yes No Yes Yes No
X-DNA comparison Yes Yes No No Yes No
Y-DNA comparison No Yes No No haplogroup label haplogroup label
mtDNA comparison Yes No No No haplogroup label haplogroup label

You must link your DNA match list and your pedigree chart and share them on the major autosomal DNA comparison websites: can not currently be recommended for genealogy for several reasons:
Add DNA information to your genealogy database:
Add genealogy information to the online DNA databases: tools tools

AncestryDNA tools

Identity v. Anonymity

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.
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.
Also take care not to link a male DNA sample to a female's pedigree chart.
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 the internal messaging system and AncestryDNA/MyHeritage/23andMe or Facebook messages only to exchange e-mail addresses.

Case studies

The blackberry baby

O'Deas of Moveen West

Further reading