A model of the chemical structure of DNA (Deoxyribonucleic acid) was first revealed in 1953 by the research work of Jim Watson and Francis Crick at the Cavendish Laboratory in Cambridge. They showed that DNA was made from four "bases" arranged in a long chain and twisted into the shape of a double helix. Each different sequence of the four bases along the chain formed the blueprint, or code, for the manufacture of a particular protein in the body. These sequences of DNA bases, capable of instigating the production of proteins, are the body's genes and they are themselves contained within 23 paired chromosomes. Every child that is born receives 23 chromosomes from each parent, carrying within them the genetic information that will give that child inherited characteristics from both parents.
Every human receives approximately 3,000,000,000 DNA bases in their chromosomes and the genes that hold the blueprints for all the proteins required to build the human body consist of 10 to 15 per cent of these three billion bases. Much of the remaining DNA has no obvious purpose and is often referred to as "junk" even though it contains the major share of the base sequence variations that differentiate one person's DNA from another's. This "junk" DNA is therefore more valuable than the genes for identifying relationships and for genealogy.
Human DNA patterns are unique to an individual (except in the case of identical twins). Half an offspring's DNA comes from each parent and, because there are so many genes in Human DNA, the chances of a DNA match between unrelated individuals is smaller than the chances of winning the jackpot in the National Lottery. The overlap of my DNA sequence with that of my grandmother or grandfather will be 25% and with each of my great grandparents 12.5% on average. The DNA overlap between first cousins will be in the region of 12.5% and with second cousins about 6.3%.
The massive Human Genome Project to "read" the complete DNA sequence for humans is expected to be completed by the year 2005, but for the identification of family relationships only small portions need to be read. This has now become a matter of routine in many laboratories which offer DNA sample testing for forensic purposes and paternity determination.
There is a special type of DNA, termed mitochondrial DNA or mtDNA, which is inherited from the mother alone. In a stable population this means that one mother will eventually pass her mtDNA to all the females many, many generations later. Using this fact to trace the descent of humans across geographic areas, genetic scientists have concluded that all present-day humans are descended from one African woman who lived between 150,000 and 180,000 years ago. This does not mean that this one African woman was the only woman alive and procreating at that time, but she was the only woman who has an unbroken line of female descent to the present day 1. On a much shorter timescale, DNA studies of the present population of three hundred or so on Tristan da Cunha, in the South Atlantic, have shown that there were five female founders back in 1817 2
Can the developments in gene mapping have an importance for genealogists in tracing ancestors since each DNA sequence is distinctive to an individual and his\her kin ? If a DNA sample from an earlier generation is available then it should be possible to trace the living decendants. Researchers in the Washington have been able to confirm that DNA can be extracted successfully from the bones of American Civil War skeletons, whilst workers at the Zoological Institute in Munich have been able to extract mtDNA from 600-year old human remains. Russian geneticists have examined a Neolithic burial site in Siberia and obtained genetic characteristics of the population circa 4,000 - 3,000 B.C. French researchers have been successful in isolating nuclear DNA from bones some 12,000 years old.
All this work suggests that there is an opportunity to confirm genealogical hypotheses by scientific methods provided human skeleton or tissue fragments are available from the deceased.
1 Wilson, A. C. and R. L. Cann "The recent African genesis of humans", published in Scientific American, 1992 Vol 266 No 4
2 Soodyall, H et al "The founding mitochondrial lineages of Tristan da Cunha islanders", published in American Journal of Physical Anthropology, October 1997, Vol 104