Genetics 101: All About DNA

In the PBS series Finding Your Roots, DNA testing is one of the many tools Henry Louis Gates, Jr. uses to tell the family histories of his guests.

In the last few years Gates, a Harvard professor with a Ph.D. in English Literature, has become the go-to guy for genetic genealogy at PBS. He has turned his fascination with his own family history into an expertise in using DNA to complete the picture of people’s ancestry. His shows don’t just offer great stories — they illustrate the power of DNA to uncover those stories. In the process, Gates explains a lot of the science.

As Professor Gates knows, just a few key concepts underlie genetic genealogy.  In this post we outline a few of those concepts so that you too can “be the expert”.

1) Your genome is contained in chromosomes. Chromosomes are long DNA molecules.  Almost everyone has two copies of chromosomes 1 through 22 (some have three copies of chromosome 21). We each have at least one X chromosome. Typically, females have two X chromosomes and males have an X and a Y chromosome.  Each of us also has billions of copies of a small DNA molecule called mitochondrial DNA.
 
2) Chromosomes are made up of DNA.  DNA is a molecule consisting of two connected strands of smaller molecules often identified with the letters A, C, G, and T.  The two strands are important because they allow a DNA molecule to be copied: the two strands come apart and a new strand is created based on each of the original strands. (Our DNA is copied as we create new cells in our bodies and when our bodies create sperm and egg cells.) In this way DNA is copied very faithfully, over and over again.  Occasionally, however, there is a “misspelling” when a new copy is made, and a mutation is introduced.
 
3) Different parts of the genome are inherited in different ways. We inherit one copy of chromosomes 1 through 22 from each of our parents. Girls inherit an X chromosome from each parent; boys inherit an X from their mother and a Y chromosome from their father.  All children inherit mitochondrial DNA just from their mother.  See video for more on DNA and inheritance:

 
4) Mutations make genetic genealogy possible. While DNA is passed from generation to generation virtually without change, there are occasional “mistakes” in the copying, which we call mutations. Geneticists can use these mutations, also known as “variants,” to mark important evolutionary divisions among humans.  While any two people’s genomes are 99.5 percent identical, that small bit of difference is incredibly important. It’s what makes each of us unique. And that little bit of variation across genomes also allows genetic genealogists to map out information about our ancestry.
 
5) Genetic genealogy is about comparisons. By comparing the DNA of two individuals, one can figure out the relationship of the two people: the more closely related two people are, the more similar the DNA. The more DNA that is analyzed, the better the estimate of the relationship. By comparing the DNA of one person to the DNA of people from many places, one can learn something about an individual’s ancestral origins:  the individual’s DNA is likely to be most similar to the DNA of people with whom he shares ancestry.

With the few concepts outlined above, and the definitions below, you too can get started on this rewarding journey of discovery!

Some Key Terms:

Autosomal DNA – DNA found in chromosomes 1-22 (the non-sex chromosomes). Autosomal DNA is passed down to us from both our mothers and fathers. Because of shuffling, or “recombination”, each chromosome is a mixture of the autosomal DNA from one pair of grandparents.
 
Chromosomes – Long strands of DNA found in the cells of all living things. Humans typically have 23 pairs of chromosomes, one of which is the pair of sex chromosomes that determine whether a person is male or female.
 
DNA — A long coiled molecule that consists of two strands in a spiral ladder-like configuration (double helix) in which each rung is a pair of smaller molecules labeled by the letters “A”, “T’”, “C”, or “G”. DNA is short for DeoxyriboNucleic Acid.
 
Gene — A stretch of DNA containing the instructions for the production of proteins and other biological molecules. There are about 30,000 genes in the human genome. Genes make up just 2% of the genome.
 
Genetic Genealogy — The use of a person’s DNA to infer a person’s heritage and confirm or discover family relationships.
 
Genome — The genetic information in an organism. A single human genome includes over three billion DNA base pairs (represented by the letters A, C, G, and T).
 
Haplogroup — A labeled set of similar DNA sequences of the Y chromosome (paternal line) or the mitochondrial DNA (maternal line). Sequences within a haplogroup share a set of mutations, and a common male (for the Y chromosome) or female (for the mtDNA) ancestor. Haplogroups are labeled with a combination of letters and numbers such as “L3a1” and “D4”. People with the same maternal or paternal haplgroup share a common ancestor in the last 1000 to 50,000 years, depending on the haplogroup.
 
Maternal Line – The ancestral lineage through one’s mother, maternal grandmother, her mother, and so on. Mitochondrial DNA tracks this lineage. Although both men and women have mitochondrial DNA, only women pass on mitochondrial DNA to their children.
 
Mitochondrial DNA — Small rings of DNA in cell components called mitochondria. Mothers, and not fathers, pass this DNA on to their children.
 
Mutations — Changes in the DNA sequence at a particular location.
 
Paternal Line — The ancestral lineage through one’s father, paternal grandfather, his father, and so on. The Y chromosome tracks this lineage. Only males carry a Y chromosome.
 
Y chromosome — The chromosome that is passed from a father to his sons. The Y chromosome includes genes that give people male attributes.

 
 
About the Authors:
Joanna Mountain is a geneticist who is a consultant to the PBS series “Finding Your Roots,” and consulted previously on the PBS series “Faces of America.” Dr. Mountain completed her PhD in Genetics at Stanford University and has spent over 20 years studying human genetic diversity. Currently, she is Senior Director of Research at 23 andMe, Inc.

Scott Hadly is a writer with extensive experience as an investigative reporter.  Currently Mr. Hadly is Content Editor at 23andMe, Inc.