Family Trees & DNA Matching
Tossed Salad, not Puréed Soup
Libby Copeland, author of The Lost Family: How DNA Testing is Upending Who We Are, calls early users of DNA websites citizen-scientists. They were driven to solve mysteries found among their DNA matches. The addition of autosomal testing (both sides, not just a father’s or a mother’s ancestral line) and accessible databases encouraged the rest of us to spit in little tubes and build family trees based on the results. Information shared by these “early seekers,” a term often used by the author, can be difficult to comprehend.
With words like recombinant and centiMorgan, I have seen the eyes of attendees at presentations glaze over. Learning is easier when we associate new knowledge to something already understood. This is especially useful when the facts are counterintuitive. I’ve heard people say: “My brother/sister and I came from the same parents, so we have the same genes.” This appears to make perfect sense, but it’s not true.
It is true that each sibling’s DNA is a combination of 50% from Mom and 50% from Dad. And, if Mom’s DNA and Dad’s too had been puréed as are the ingredients in my delicious Sweet Potato Soup, siblings would have the same genes (up to a point—more about this later).
Without divulging secret details, the recipe calls for sautéing sweet potatoes, onions, and garlic in broth, dumping the softened mixture into a blender along with fresh ginger and blasting away on high. When served at a dinner party, my guests receive a cup of exactly the same amount of each ingredient.
But DNA is not like puréed soup. First the 50% that one sibling gets from a parent is different from the 50% another sibling gets. A better way to visualize the structure of DNA is as a tossed salad— pieces not liquid. Salad tongs are used to serve the guests—each bowl will vary. One overflows with cherry tomatoes, another has more cucumber but less lettuce, and someone missed out all together on the zucchini. (That cousin didn’t get any of his fourth-great-grandma’s DNA.)
Among close relatives (first cousins, aunts/uncles, grandparents), the relatively high percentage of shared DNA usually won’t cause consternation. The differences become apparent in more distant relationships. This is the reason sibling A can share DNA with a fifth cousin, while sibling B doesn’t. But both siblings are biologically related to that fifth cousin via their fourth-great-grandma (their most recent common ancestor).
Mentioned earlier was the suggestion that if DNA resembled puréed soup, siblings would have the same DNA. In a sense, identical twins are a result of this concept. Mom and Dad each contribute 50% of their genes to an egg and a sperm, respectively. When the egg and sperm join and the DNA recombines to form a zygote, then splits to form two embryos, the siblings are “identical.” The “up to a point” is the environment. Even before babies are born, their genes are affected—some will turn on, some off. Identical twins may not, in rare cases, be the same sex. But this is another matter—let’s get back to the zygote and single-births.
The genes a child inherits from each parent create a unique baby. Recombinant DNA is a new pattern of genes, no longer half Mom and half Dad. The different fifty percentages from each parent—randomness—is inherent in the process. Don’t get too excited about being unique. Of our working DNA, we share 98.5% with chimps and bonobos and 97.5% with mice. Only .1% (one-tenth of one percent) distinguishes us from each other.
Now we arrive at measuring DNA—which is the basis for all the ancestry-relative-matching websites. Since DNA is not like soup, the measuring unit can’t be ounces. Using salad ingredients as an example first requires imagining each vegetable piece as having the same dimensions—except for length. CentiMorgans—the unit of measurement of DNA is the length (actually distance) of gene segments which biological relatives share.
Your “shared matches” have lengths of genes at the same distances on a specific chromosome. Depending on the chart used, a parent and a child “share” ~3330–3720 centiMorgans, while those fifth cousins share 0–99 centiMorgans. For more details and purely scientific explanations of the nature and measurement of DNA, download the Resource List.
I submitted my DNA toward the end of 2016 just as the number of customers for Ancestry.com's kits began to rise exponentially. When my results arrived, I could have known within days that I didn’t share DNA with the daddy who raised me. But it took much longer to emotionally accept the fact.
Using “shared matches,” by mid-2017 I had identified my biological father. In communicating with these DNA matches—relatives—I realized that many didn’t know how to use the website’s features to their own advantage. They could find elaborate instructions and technical explanations but not a concise start-to-finish manual with examples. I created My Surprise Family: Find Your Ancestry Story to help them. My book is a mystery, a memoir, and a manual. My two-course soup and salad meal is one of many “how to” anecdotes.
Would love to hear if this explanation of DNA construction and measurement was useful to you. Email: firstname.lastname@example.org or comment below.
See Genealogy link on the home page for longer articles.
For those interested in the history of and the emerging industry surrounding DNA testing, I do recommend The Lost Family: How DNA Testing is Upending Who We Are. Ms. Copeland covers many topics including the controversial use of public DNA databases to track down those accused of crimes. Stories about people who have discovered surprises in their family trees—with satisfying or sad results—are used to question how we define ourselves and our relationships.