The mysteries of DNA and how it can help you
in your genealogy research.
by Janet Schaible
What is DNA?
DNA, or deoxyribonucleic acid, is something each of us have in every cell in every part of our body with only a few exceptions. (Our red blood cells lack DNA, while the white cells contain
it.) Shaped like a twisting ladder, or what scientists call a double helix, DNA is formed of four chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The way these bases fit together (A only fits with T and C only fits with G) form units called
base pairs (AT, CG). Each base pair make up a step or rung of the twisting ladder. The order of these bases is where the information is read, in much the same way computer binary (1's and 0's) are what the computer reads to make this page. Human DNA consists of around 3 billion bases, and more than 99 percent of those bases are the same in all people.
Small segments of DNA, called genes, are the blueprint that tells cells how to grow and form and what they will be. The small amount that are not the same are the genetic instructions that make you and me different.
A small amount of DNA can found in a structure of our cells called the mitochondria but most DNA is contained in the nucleus (nuclear DNA) of our cells in groups called chromosomes.
Chromosomes are the little paired packages of DNA that transmit our genetic information. In humans there are 23 pairs of chromosomes. 22 of these pairs are basically the same, one half of the pair deriving from the mother, the other from the father. The 23'rd pair is different. If both halves of the pair is the same it is called an X chromosome and if they are different it is a Y chromosome. It is these chromosomal differences which programs an early embryo to develop as a male or female. Every woman has two X chromosomes. Every man has an X chromosome and a Y chromosome. The whole package of DNA
in our cells, both the chromosomes within the nucleus and the DNA in mitochondria, is called a genome.
How can DNA be useful in genealogy?
For the most part, our genome is a shuffled combination of DNA passed down by our ancestors. Fortunately there are small segments of DNA called genetic markers that are only occasionally changed
by mutations and they are ones that are most interesting to genealogists.
Genetic genealogy is very useful if you have hit that good old "brick wall" and don't know where to look next, or you belong to a group that do not have traditional genealogical records.
It will not tell you specifically which ancestor you come from, or even if you are related to other family members unless they have DNA on record, but it might tell you where to look next if some other persons genetic markers are close to yours.
It may be possible to find relatives you never knew about in growing databases of genetic markers such as Ancestry.com DNA
 which adds test results to family trees and shows close matches in others. Even without a close match, DNA mapping will still tell you which limb of the tree your branch is on.
Y chromosomal DNA is passed down exclusively from father to son and is used to trace paternal lineage. On rare occasions there is a mutation or change in one of the markers of the Y chromosomal
DNA. These mutations occur at random and always occurs as an insertion or deletion of part of a marker.
Mitochondrial DNA (or mtDNA), passed down exclusively from mothers to their children of both sexes, allows researchers to trace a maternal lineage far back in time. Both Y chromosomal DNA
and Mitochondrial DNA testing results can be divided into genetic population groups called haplogroups. "Journey to 10,000 BC," a very interesting TV show that the History Channel repeats every so often, is a perfect example of an ancient haplogroup migration. This one deals with the first North American migration and what the traveling group might have encountered.
Map of Y & mtDNA haplogroups before the European expansion beginning about 1500 AD.
Next week-The different kinds of DNA testing and what they mean to you.
Glossary of DNA terms:
Base Pair: Two chemicals (bases) that form a link in the DNA double helix. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.
Chromosome: the DNA bundle that carries portions of the hereditary information in the nucleus of a cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs or 46 chromosomes.
DNA: the chemical the genetic instructions inside the nucleus of a cell.
Gene: genes are pieces of DNA passed from parent to offspring.
Genetic Markers: small segments of DNA with identifiable locations on a chromosome.
Genome: all the DNA contained within a cell.
Haplogroups: large population groups that migrated within and out of Africa over 60,000 years ago to different parts of the world.
Mitochondria: structures within cells that convert energy from food into a form that cells can use. The Mitochondria also contains a small amount of DNA passed down from the mother and remains basically the same as it was thousands of years ago.
Mitochondrial substructure
Mutation: a permanent structural alteration in DNA.
Nuclear DNA: the DNA in chromosomes that are inherited from both parents.
Nucleus: structures within cells that houses the chromosomes. Structure of a nucleus (.pdf
file)
Y Chromosome: one half of the pair of chromosomes that determine the sex of a child. Only carried by males the Y Chromosome is passed from father to son virtually unchanged for generations
except by random mutations.
Further Information on Genetic Genealogy:
Atlas of the Human Journey - National Geographic
Explorations of ancestral human migration patterns - National Geographic
Genetic connections among organisms - the Tree of Life project
Tracing Ancestry with MtDNA - NOVA (PBS)
International Society of Genetic Genealogy (ISOGG) - The first society founded to promote the use of DNA testing in genealogy.
Human Genome Project Information - an international effort to discover all the approximately 30,000 to 35,000 human genes
in the human genome and make them accessible for further biological study.
Why Y? The Y Chromosome in the Study of Human Evolution, Migration and Prehistory - modern genetic analysis for exploring the role of fathers in human history.
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