DNA is the key to life - is it the key to family history too?
This page is an attempt to clarify my thoughts on DNA. In late 2008 I won a voucher for a substantial discount on a DNA test in a Family Tree Magazine competition and wanted to know more about DNA - so this is a result of my research. Wiki has a good introduction to genetics as does the Blair Project (don't be afraid - it only shares the surname with the our ex-PM!) Also there are some vidoes on the Family Genetics Site - click "Enter" on relationship testing, then they're on the menu on the right. Blaine Bettinger also has an informative ebook..
What is it?
"Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses." says Wiki. It contains all the instructions for making a life form. The Genome is a term used to describe the whole DNA sequence - and is broken down in humans into 23 chromosomes pairs and Non-coding_DNA. There are about 3 billion base pairs (see below) in the DNA from a single human cell and it has a length of ~1.8 m (but at a width of ~2.4 nanometers).
Who discovered it?
The substance was first isolated in 1869 by a Swiss physician Friedrich Miescher. The structure was first described in 1953 by James Watson and Francis Crick, using unpublished X-ray data obtained by Rosalind Franklin who was working with Maurice Wilkins
DNA has a double helix structure - the 2 strands of the helix are held together by bonds between "base-pairs" - the bases being adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T) - A bonds with T, C with G.
Y-DNA, mtDNA and Autosomal DNA
In fact the human body has 2 sorts of DNA - nuclear DNA found in the nuclues of the cell, and mitochondrial DNA found in the mitochondria of cells. Both sorts are used in genealogical tests. The Y chromosome is passed from father to son - the mother doesn't have any as she has 2 X chromosomes - so Y-DNA represents the paternal line. Mitochondria on the other hand are contained in the tail of the sperm, which is left outside the egg at fertilisation - so mtDNA represents the maternal line.
Autosomal DNA is the non-gender stuff, (not X or Y) that combines and recombines (blue/brown eyes etc.) This can be tested to shown relationships back (currently) to 2nd great grandparents. There are also tests which can apparently give an indication of ethnic make-up, although I'm not sure how reliable this is yet.
DNA testing invented in Leicester
The use of DNA for identification purposes was first reported in 1985 by Alec Jeffreys at Leicester. This is the basis of forensic DNA testing. The linked article also describes some of the techniques used for extracting information. From these processes use of DNA for genealogical purposes began.
Bryan Sykes, & daughters of Eve and Cheddar Man
After analysing mtDNA, in the book published in 2001, The_Seven_Daughters_of_Eve, Brian Sykes postulated the theory that European women descend from 7 clan mothers. More recent work has developed this theme and some suggest there are more than 7 as sub clans are identified. He was one of the first to extract DNA from ancient bone - for example in the case ofthe man from Cheddar Gorge. His later book, The Blood of the Isles is his account of the settlement of Britain. Both books are very readable.
Haplotypes, alleles, markers, STRs, SNPs
The details of the process are described better elsewhere see Wiki - what follows is a short summary.
In the "male family relationship" tests, segments of Y-DNA are examined for genetic markers where short groups of base pairs repeat. These are patterns of DNA known as Short Tandem Repeats STR. These repeats cause the copying process a problem and sometimes a mutation occurs and a different number of repeats is generated. These repeats have numbers known as DYS - DNA Y chromosone Segment numbers. The number of repeats at a marker is known as an allelle. The collection of allelles is known as a haplotype. There can be different numbers of markers tested depending how much you pay! Common examples are a basic 12, more common 25 or 37, and in depth in excess of 40. The number of markers that match determines the probability of a genetic match.
Nicholas Poulin has a nice little map of the section of the Y chromosome. I'm not convinced about his genetic links however!
Whilst the haplotype gives an indication of genetic origin, this is better determined by testing the SNPs (Single Nucleotide Polymorphism) which mutate at a slower rate than the STRs and are used to create the "phylogenetic" trees of the haplogroups - see below. The SNPs are tested in the "male deep ancestry" tests, but as more SNPs are discovered, these tests may need to be repeated.
Mitochondrial DNA is tested in the female test, but as SNPs in the mtDNA are tested, this gives a "deep ancestry" test, resulting usually in the allocation to one of Bryan Sykes "Daughters of Eve" - or their non-European counterparts
For comparison of what different companies test see the excellent GENDNA chart
What can DNA testing tell us?
One of the major problems with DNA testing is that it is different from other genealogical evidence. The latter tends to be black or white - a marriage certificate states the bride's (or groom's) father's name. It is a fact! DNA results however are a probability. On the other hand, given a reputable test, DNA tests don't lie, whereas conventional evidence is often black when is appears white (eg. bride's age on a marriage certificate isn't always correct!). So it needs very different mindset for interpretation.
For example, here's a quote from the DNA-Newbie Rootsweb list:
To give you one example, a lady came to me saying that she KNEW her ancestor (with a VERY GERMAN name and found in American German Lutheran church records) had to have been English and not German. When I asked her why she KNEW that she replied that the haplogroup her male relatives fell into was almost exclusively English and not German. So I did some checking and learned that while the majority of people who fell into this haplogroup were English, 50% of Germans were as well. I think people rely TOO heavily on the "science" and "odds" and tend to ignore the factual evidence -- and that is the caveat I'd offer with regard to DNA testing and the results.
The first thing to note is the haplogroup - the group to which the tested haplotype belongs. This indicates genetic origin - which route out of Africa did the ancestor take. YDNA and mtDNA maps show the relative distributions of the haplogroups and the projected migration routes.
There's a wonderful interactive map at the Bradshaw Foundation
The tree of haplogroups is complex but the principle is simple. A mutation in one of the SNPs indicates where a new haplogroup formed - so about 55,000 years ago haplogroup BT split from group A - to quote from the very complex tree on isogg (which incidentally has accounts for each of the haplogroups).
Prof. JD McDonald published maps showing the distribution of haplogroups - well worth a study!
The BT haplogroup split off from haplogroup A 55,000 years before present (bp). It probably appeared in North East Africa. The C,FT haplogroup was the common ancestor of all people who migrated outside of Africa until recent times. The defining mutation occurred 31-55,000 years bp in North East Africa and is still most common in Africa today in Ethiopia and Sudan. The DE haplogroup appeared approximately 50,000 years bp in North East Africa and subsequently split into haplogroup E that spread to Europe and Africa and haplogroup D that rapidly spread along the coastline of India and Asia to North Asia. The IJ haplogroup characterizes part of the second wave of emigration from Africa that occurred via the Middle East 45,000 years bp and defines two branches I and J that emigrated northwards and eastwards into Europe. The J branch subsequently split again and contributed to the current North African population. The NO haplogroup appeared approximately 35,000-40,000 years bp in a region east of the Aral sea; subsequent branches spread to North Asia (N) and another branch (O) to South Asia via North India.
Wiki has a good explanation
This is a diagram of the phylogenetic tree from YCC
If you're interested in the details of the markers and their mutations, the YCC has a table
Diana Gale Matthieson's chart is even better.
Secondly, the closeness of a match (ie. how many allelles
match) with another person gives a probability of having a common ancestor. For
an accurate assessment there's a
Time to MRCA
calculator. FTDNA have a graph showing the probabilities
Further Interesting Links
Glossary from FTDNA
glossary from isogg
YBase DNAHeritage database
Ysearch FTDNA database