Genetic Genealogy is an exciting and rapidly expanding complement to traditional (documentary) genealogy. Using genealogical DNA testing to determine (or at least, infer) the level and type of the genetic relationship between individuals, it can be a powerful tool in tracing one's lineage.
The non-profit International Society of Genetic Genealogists (ISOGG) advocates for and educates about the use of genetics as a tool for genealogical research while promoting a supportive network for genetic genealogists. They provide some fundamental information on how DNA is used as follows:
What is DNA?
A representation of the DNA double-helix
"A polymeric molecule made of deoxyribonucleotides, hence the name deoxyribonucleic acid. Most often has the form of a "double helix", which consists of two paired DNA molecules and resembles a ladder that has been twisted. The "rungs" of the ladder are made of base pairs, or nucleotides with complementary hydrogen bonding patterns. "
Translation: DNA is the molecule that encodes the genetic instructions for building and operating all living things. Humans are 99.9% genetically identical. That is a very important concept to remember. All the differences we see in the way people look, what diseases they may be prone to etc. come from the .1% difference. That doesn't seem like much until you realize that there are about THREE BILLION base pairs in which those differences may be expressed. The human genome is the complete set of human genetic information. It is located within the 23 pairs of chromosomes. Each half of the pair represents our mother or our father. These 23 chromosomes reside within the nucleus of our cells. There is also a small DNA molecule found within individual parts called mitochondria.
Simplified human cell
Your DNA is in almost every cell that is you. In the above diagram the cell is the black circle. The nucleus is the green circle. The purple twisted matter within the cell nucleus is DNA. The orange circle is your mitochondria which also contain DNA. The DNA replicated in the cells of your body is essentially your personal building and operating instructions given to you by your parents. But more than that it contains the encyclopedia of where you came from. It is the story of your ancestors told through the bits of DNA that they have passed down to you. It is that passing down, generation after generation that makes it so important to genealogists.
What Kind of DNA do Genetic Genealogists Use?
There are four different kinds of DNA that genealogists use in different ways for obtaining ancestors’ information relevant to genetic genealogy. We have 4 different kinds of DNA available to us because of unique inheritance patterns for each kind of DNA – meaning we inherited different kinds of DNA from different ancestral paths. If one kind of DNA doesn’t work in a particular situation, chances are good that another type will.
Genetic genealogy makes use of 4 different types of DNA.
Y DNA – passed from males to male children, only (your patrilineal, or father’s paternal line). Because each man inherits this Y-DNA virtually unchanged from his father and his father from his father (and so forth) Y-DNA is the easiest DNA to use for genealogical purposes.
Mitochondrial DNA – (abbreviated mtDNA) passed from females to both genders of children, but only females pass it on (your matrilineal, or mother's maternal line)
Autosomal DNA – (abbreviated atDNA or more rarely, auDNA) the 22 chromosomes that recombine during reproduction.
X Chromosome (the sex chromosome) – A female child receives one X-chromosome from her father and one X-chromosome from her mother. A male child receives an X-chromosome from his mother and a Y-chromosome from his father.
Why the Y?
The human Y chromosome is a male-specific sex chromosome. Nearly all humans who possess a Y chromosome will be morphologically male. Although the Y chromosome is situated in the cell nucleus, it only recombines with the X-chromosome at the ends of the Y chromosome; the vast majority of the Y chromosome (95%) does not recombine.
When mutations (errors in the copying process) arise in the Y chromosome in the form of single-nucleotide polymorphisms (SNPs) or short tandem repeats (STRs), they are passed down directly from father to son in a direct male line of descent.
Patterns in mutations reveal specific haplotypes, the term for the set of numbers that consists of your Y-chromosome or mitochondrial DNA results. A haplotype is also known as a genetic signature or a DNA signature.
Haplogroups are a group of similar haplotypes that share a common ancestor with a SNP mutation. Because a haplogroup consists of similar haplotypes, it is possible to predict a haplogroup. A SNP test confirms a haplogroup. Haplogroups are assigned letters of the alphabet, and refinements consist of additional number and letter combinations. Example: R1b1a. Y-chromosome and mitochondrial DNA haplogroups have different haplogroup designations. Haplogroups pertain to your deep ancestral origins dating back thousands of years.