Genetic Genealogy

Irish Family & Social History SO03

6 p.m. Tuesday 2 February 2021 and Tuesday 9 February 2021

Zoom lecture

by Paddy Waldron

WWW version:


2 February (Passcode required); 9 February (TBA)


Review of beginners' session

male offspring female offspring
sperm Y chromosome X chromosome
22 paternal autosomal chromosomes
egg X chromosome
22 maternal autosomal chromosomes
DNA component Inheritance path Inherited by
Y chromosome From father only (and only if male) males only
autosomal chromosomes (autosomes) Equally from both parents everyone
X chromosome(s) Unequally from both parents males x1, females x2
mitochondrial DNA From mother only everyone

Identity v. Anonymity

The trade-off

Basic rules

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:
A woman does not have a Y chromosome, so may ask a male relative with the relevant surname to swab:
    a father, brother, nephew, cousin, etc., if her interest is in her maiden surname; or
    a husband, son, brother-in-law, father-in-law, etc., if her interest is in her married surname.
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.
You must NOT attach a female name or a female's pedigree chart to a male DNA sample (or vice versa), as this causes untold confusion.
Be especially careful not to inadvertently link a male's Y-DNA results with a female's autosomal DNA upload 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.
Use a photograph:
If you upload a photograph (or any image) to your AncestryDNA account before you receive your initial results, then the photograph (hyperlinked to the match details) will appear on the AncestryDNA Insights page of all your matches for as long as you remain in their eight newest matches with photographs.
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/LivingDNA or Facebook messages only to exchange e-mail addresses.

Managing your matches

Fishing in all the gene pools

There are a growing number of DNA comparison websites and those interested in finding long-lost relatives should be in all of them, especially the largest ones.

Remember Murphy's Law of Genetic Genealogy, coined  while I was helping an adoptee who is married to a Murphy:

If there are N DNA comparison websites and your DNA is in N-1 of them, then your most important match will be in the Nth.

In the words of another widely used metaphor, there are many online gene pools out there and there are many people who are in only one or two of them; for maximum effect, particularly if you are trying to find an unknown ancestor who has left no paper trail, you must fish in all of these pools.

You must spit for the websites which do not allow data uploads:

You must download your data file from the website of whichever laboratory you use and upload it to the websites which do allow data uploads:
You must link your DNA match list and your pedigree chart and share them on the major autosomal DNA comparison websites:
As of 2 February 2021, I have:
Both larger databases and less strict matching criteria result in more matches.
Add DNA information to your genealogy database:
Add genealogy information to the online DNA databases:

AncestryDNA tools tools tools

Using autosomal DNA shared matches, triangulation and phasing

A counter-example

Three marriages in the United Church of England and Ireland in Kilkeedy, County Limerick: However, these triangular marriage patterns are very rare.

Shared, or In Common With (ICW), matches

A group of three or more individuals who all meet the relevant matching criteria with each other are likely to share a recent common ancestor (or, more often, ancestral couple).

When I find a new match, I am usually anxious to identify the most distant known ancestor through whom I am related to the new match.

The matches that I share with the new match are usually the first clue to solving this puzzle.

All of the DNA comparison websites allow one to identify the shared matches of two individuals in some form or another.

The matching criteria vary from one DNA comparison website to another.

The stricter the matching criteria, the more significant the shared matches.

FTDNA Family Finder

To find the shared matches of two individuals who match each other:

You will eventually identify a group of individuals, all of whom you suspect descend from a single common ancestor (or ancestral couple).

To see whether up to 10 individuals who match you also match each other:

To find the shared matches and shared cM of two or more individuals who belong to the same project, whether or not they match each other:


On each match page, there is a Shared Matches link. (Screenshot.)

The Shared Matches are those with Shared DNA of 20 cM or more with both individuals.

So C can appear in the shared matches of A and B even if B does not appear in the shared matches of A and C (if C shares more than 20cM with A but B shares less than 20cM with A).

Matches are sorted by closeness of relationship to the logged-in individual.

A can see that B matches C but cannot see the cM shared by B and C.


When the Review DNA Match page eventually completes loading, the Shared DNA Matches section:

Matches are sorted by the sum of the centiMorgans shared with the two individuals. 

A can see not only that B matches C but also the cM shared by B and C.

A's shared matches with B will be exactly the same as B's shared matches with A, in the same order.


To find the shared matches of two individuals, whether or not they match each other, use the "People who match both, or 1 of 2 kits" tool on the main menu.

This lists shared matches of Kit 1 and Kit 2, but the user sets the cM threshold of largest segment and cM threshold of total matching segments.

Matches are sorted by closeness of relationship to Kit 1.

To sort by closeness of relationship to Kit 2, re-use the tool with the kits in reverse order.

If you login in one browser tab, and open the Multi Kit Analysis menu in another tab (via a hyperlink or bookmark), then you can run an Autosomal Matrix Comparison on up to 100 kits.

While the FTDNA user matrix shows only whether or not kits are deemed to match, the FTDNA administrator matrix and the GEDmatch matrix shows the shared centiMorgans.

Triangulated matches

Triangulation and phasing are really opposite sides of the same coin.  If V is half-identical on the same region with W and Z, then there are two possibilities:

  1. W and Z are half-identical to each other on this region, in which case V, W and Z probably inherited an identical segment in this region from a single common ancestor and the relationship can be described as triangulated; or
  2. W and Z are not half-identical to each other on this region, in which case V is probably related to W on V's paternal side and V is probably related to Z on V's maternal side, or vice versa, and V's autosomal DNA in this region can be phased.

The ADSA tool by Don Worth at DNAGedcom provides a graphical representation of triangulation and phasing.

Triangulation groups

The ultimate objective is to collect DNA matches into triangulation groups. A triangulation group is a set of three or more people who are all half-identical to each of the other group members on overlapping regions.

The more individuals who are added to the triangulation group, the smaller the overlap may become.

A triangulation group of three or more individuals are very likely to share a recent common ancestor (or, more often, ancestral couple).

The triangulated matches that I share with a new match are usually the second clue to identifying through which of my most distant known ancestors I am related to the new match. Some of the DNA comparison websites allow one to identify the triangulated matches of two individuals in some form or another.

FTDNA Family Finder

One had to be a little devious to find triangulated matches directly at FTDNA.

The Linked Relationship feature was designed to identify matches who triangulate with known relatives, and then dump them all together again in paternal and maternal buckets.

You may want to have two Family Finder kits, e.g. a kit based on swabs sent to FTDNA with a full pedigree chart for paternal/maternal phasing; and a kit based on an autosomal transfer from another laboratory with a minimal pedigree chart for identifying triangulated matches (e.g. my B95575).


AncestryDNA refuses to provide any way of identifying triangulated matches.


MyHeritage shared match lists include a symbol identifying which of these matches are triangulated.

Make sure that you have not opted out of showing shared segments.

There appears to be no way to filter the list of shared matches to show only the triangulated matches.


The basic tools are free to all users:

Access to these Tier 1 tools costs USD10/month:

Other third party tools and websites

The major DNA websites do not like the load imposed on their servers by the more powerful tools designed by third party developers.

Y-DNA and surname projects

Further reading