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African American Lives
Analyzing the Evidence
The Science and the Investigators
Who am I? A Genealogy Guide
Sharing Stories
For Educators
About the Series

The Science and the Investigators
Intro Race and Science The Tests Learning from DNA
Detail of a computer screen displaying DNA sequence data.

Once you sign up for any type of genealogy DNA test, you receive a kit which includes instructions on how to obtain samples of your DNA (a simple, painless procedure which usually involves using swabs to scrape "buccal" cells from the inside of your cheek). You mail these samples back and results are returned to you (usually within 4-6 weeks) along with further data about how to read the results. Some companies may also offer the services of professional genealogists to further contextualize your findings. You can decide if you want the laboratory to store your DNA sample or destroy it.

So how do researchers use this sample to reconstruct millennia of human evolutionary history? What is it these different tests are looking at -- and looking for?

DNA (deoxyribonucleic acid) is the molecule which carries the code of chemical instructions, inherited from both parents, that determines how an organism will develop. DNA determines not just the general form of an animal or plant, but determines (at least to the extent that such things are not affected by environmental and social causes) the observable traits and physical and behavioral characteristics particular to an individual.

The DNA molecule is arranged in a double helix (like a spiral staircase or a twisted ladder). Each "rung" of the ladder is made up of a "base pair" including two of four basic chemical units: A, adenine G, guanine C, cytosine T, thymine. (In base pairs, A always combines with T, and G always combines with C, which keeps things organized.)

An organism's DNA contains a massive amount of information. There are about 3 billion of these base pairs in the human genome (our DNA considered in total). In the human genome, actual genes -- sequences of bases that actually contain hereditary instructions -- account for only about 2 percent of human DNA. Humans, according to the latest research, only have some 20-25,000 genes. The balance of our DNA is referred to as non-coding or "junk" DNA, and its uses and functions are still being researched.

The Human Genome Project, completed in 2003, identified all of the genes in human DNA and determined the sequence of chemical bases. The Anderson or Cambridge Reference Sequence, begun in 1981 and modified in 1999, mapped all the sequences of the human mitochondrial genome, using a European woman as a model. The completion of these databases have paved the way for DNA testing to determine genetic ancestry -- they are the benchmarks against which differences linked to population groups are measured.

This kind of DNA is the master codebook for the organism, and is found in the cell's nucleus, or core, and is arranged in long, coiled strands called chromosomes. Almost every cell (except for red blood cells which have none and reproductive cells in the sperm and the egg which have only a single set of chromosomes) has 46 chromosomes, organized in 23 pairs: 22 numbered pairs or "autosomes" (autosomes are non-sex chromosomes; they are numbered according to size, 1 being the largest); and 1 pair of sex chromosomes, known as "X" and "Y" one inherited from the father, one from the mother, which determine gender.

Admixture testing -- which tries to establish overall geographical and genetic heritage -- looks at many points in the genome, so researchers in that field work with autosomal DNA. It is the sex chromosomes that most interest researchers working to trace particular human lineages.

Females inherit two X chromosomes; males inherit one X chromosome from their mother and one Y chromosome from their father (the role of the male in sexual reproduction is the determination of gender -- the chromosome you inherit from your father is the determining factor). The Y chromosome is only present in males; a segment of this chromosome, called "NRY" is exceptionally stable, and is passed down without change from father to son -- its stability over time makes it a valuable tool for researchers tracing patrilineal ancestry.

Some 1,700 mitochondria are found in each cell, where they act as power sources, producing the energy that cells need to function. Mitochondria are organelles (structures that function as the cell's organs) found in the cytoplasm (the fluid outside of the nucleus that functions as the cell's "flesh"). Mitochondria -- which are thought to have evolved from primitive bacteria, independently from the cell itself -- have their own DNA, referred to as "mtDNA," which is a fraction of the size of nuclear DNA -- only about 16,000 base pairs, defining some 37 genes.

During reproduction, the male sperm abandons its mitochondria upon fertilization, so our cells contain only mtDNA inherited from our mothers. This mtDNA is passed down through the generations, without any recombination, from the mother to male or female offspring. Barring slight mutations over time, your mtDNA is identical to your mother's which is identical to her mother's and so on.

Because of this, mtDNA makes a great tool for tracing matrilineal ancestry and -- because it changes so little over time -- for plotting the history of human migrations.

Dr. Fatimah Jackson at work in her laboratory Dr. Mark Shriver and AFRICAN AMERICAN LIVES host Henry Louis Gates, Jr.
Lineage Testing These tests trace your lineage either along the male line, (using the Y chromosome) or the female line (via mitochondrial DNA). During the making of AFRICAN AMERICAN LIVES, Dr. Rick Kittles of Ohio State University, Dr. Peter Forster of the University of Cambridge, and Dr. Fatimah Jackson of the University of Maryland conducted these tests.

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Admixture Testing This type of testing analyzes a selection of specific regions of your DNA, compares them against a database and estimates what proportions of your genetic ancestry may originate from different population groups. For this part of the project, AFRICAN AMERICAN LIVES worked with Dr. Mark Shriver of Penn State University.

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