Watch "How DNA can help your Family Tree Research" - a video of a presentation we gave at the "Back to Our Past" exhibition at the RDS, Dublin on 13th Oct 2012. Just click here.

With Y-DNA testing we are only looking at one particular line of an individual's ancestors - specifically the patrilineal line of the participant, their father's father's father etc (the blue line on the far right in the diagram). In other words, we are attempting to trace the participant's Y-chromosome back to its origin. This is why Y-DNA testing is only done on men bearing the Spearin name.

The Y-DNA test[1] can also tell us what route our patrilineal ancestors took out of Africa, what mutations occurred and when, and where they stopped along the way. These migratory patterns of the different Y subgroups (haplogroups) go back to the common male ancestor of all modern humans, an African man who lived 60,000 years ago. There are two haplogroups emerging for the Spearin clan - one is haplogroup I1c (also known as I2b) and the other is R1b1a2. The migration of these haplogroups out of Africa is shown in the map below.

Another type of DNA test can trace the matrilineal line, the mother's mother's mother, etc. This is done via mitochondrial DNA testing[2]. The sperm cell is all head and no body (i.e. all nucleus and very little else) and thus contains no mitochondria, so we will have inherited all the mitochondria in our cells from our mother, and her mother, and her mother's mother, etc. Mitochondrial DNA testing has elucidated migratory patterns for the different mitochondrial subgroups, going back to the first common female ancestor of all modern humans, an African woman who lived 150,000 years ago. However this test will not help us determine the origins of the Spearin name (which is passed from male to male).

A third type of test can help identify more recent relatives (up to 5^th cousins, which represents a common ancestor up to about 200 years before the participants birth). This is the Family Finder test[3] and it estimates the percentage of autosomal DNA[4] shared between two individuals. And this brings us back to a topic we mentioned in the last section, namely 'recombination'.

During meiosis (i.e. the production of the sex cells), each of the 46 chromosomes assemble into their chromosome pairs (23 in total). In the animation, the red and green chromosomes are one pair, and the pink and blue ones are another. They replicate, and produce a copy of themselves (so that each pair becomes 4 chromosomes). And this is where the weird bit happens. Before splitting apart into separate cells, they actually swap parts with each other (Red: I'll swap my left foot for your left foot, okay? Green: Deal!).

This exchange of genetic material between chromosomes is completely random. If this 'swapping' didn't happen, then each chromosome in each pair would be passed on unchanged to the next generation. And since we inherit one chromosome in each pair from our father and the other from our mother, this would mean that our child would inherit either his grandfather's or his grandmother's chromosome intact ... but it doesn't happen like that. Instead our child inherits a hybrid or mongrel chromosome, containing genetic material from both his grandfather and his grandmother (i.e. his grandfather's body with his grandmother's foot). Thus, over the generations, the contribution from any individual will be diluted by this process of recombination.

The Family Finder test looks at how much genetic material is shared between two individuals and calculates if they are likely to share a common ancestor. However the test is only accurate back about 7 generations (i.e. to your 4th great grandparents). This is because with each generation the contribution of an ancestor becomes more and more diluted.

Put another way, you share 50% (1/2) of your 30,000 genes with each of your parents, approximately 25% (1/4) with any one of your 4 grandparents (remember, recombination may mean you have 20% from one, 30% from another, etc), 12.5% with a great grandparent (1/8) , and only 1.6% with a 4th great grandparent (1/64). Thus, after 7 generations, the contribution of any single ancestor is too diluted to be detected reliably. Siblings will share roughly 50% of the same genes, first cousins 12.5%, third cousins, 0.78%, and fifth cousins 0.05% (which is about 15 out of the 30,000 genes in the human body). However, random recombination means that two siblings could theoretically share no genetic material at all.

It is now possible to calculate which piece of genetic code was inherited from which grandparent. Thus if you match someone at the level of 4th cousin, it is possible to identify which of your 4 grandparents you are likely to share in common. This helps focus further documentary research on the correct family line. However, the calculation requires that one parent and 3 of your siblings are also tested.

It is also possible to characterise your ethnic origin with the Family Finder test and to estimate the percentage of your genes that are European, African, Asian, and Native American in origin. I am disappointingly 100% European so my hopes of finding an exotic ancestral connection have been effectively dashed.[5]

There are other DNA tests that don't concern us here, such as paternity testing, and tests for markers of diseases that may be wholly or partially influenced by genetic inheritance.

For a complete list of companies that offer DNA testing, visit the ISOGG website (International Society of Genetic Genealogists) at http://www.isogg.org/wiki/List_of_DNA_testing_companies.

[3] See also http://www.familytreedna.com/faq/answers/default.aspx?faqid=8#879. Other companies use different names to describe their own particular autosomal test.

[4] That is, the DNA in the remaining 44 chromosomes when one excludes the sex chromosomes X and Y.

[5] Actually, this is not entirely true - there is still a slim chance but I can't go into it here!