Finding Long-lost Cousins Using DNA

Ennis Mens Shed

4:30 p.m. Wednesday 22 March 2023

by Paddy Waldron

WWW version:




No eating, drinking, smoking, chewing gum or washing teeth for one hour before swabbing or spitting, in order to ensure an uncontaminated DNA sample.


Components of DNA

The basic science

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.
By the time we get to 256 GGGGGGgrandparents, there will be some people who are genealogical ancestors but not genetic ancestors.
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.

DNA comparison

Y-DNA and surnames

Y-DNA follows the same inheritance path as is typically followed by surnames.

In principle, your match list should contain dozens of men with your exact surname.

In practice, there are many reasons why this may not be the case:

A surname/DNA switch is defined as the use of a surname different from that used by the genetic father, which may be:

Surname/DNA switches are just one cause of surnames having multiple Y-DNA signatures. Many surnames, particularly occupational surnames and surnames in countries which have had many immigrants speaking one or more foreign languages, have multiple independent genetic origins for more mundane reasons.

Conversely, two different surnames can have the same Y-DNA signature if the common ancestor lived before the adoption of surnames, about a millenium ago.

Among the myriad of, possibly one-off, circumstances causing surname/DNA switches are:

For example, Osman Wilfred Kemal's mother died in childbirth in 1909 and he was brought up by his maternal grandmother Margaret Hannah Brun née Johnson and became known as Wilfred Johnson. Wilfred Johnson's grandson, known as Boris Johnson, became Prime Minister of the United Kingdom in July 2019. Prime Minister Johnson has Kemal Y-DNA, but a surname that was not used by any of his eight greatgrandparents and that descended via a female GGgrandparent. See here. hosts three types of DNA projects, co-administered by people like me:

While awaiting your results, start typing up your family tree, using, for example:

Once you have your Big Y-700 results:

If USD379 (EUR351 at today's exchange rate) is beyond your budget:

Autosomal DNA and cousins on all sides

Fishing in all the gene pools

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:

Further reading