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:
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.