What do you really learn from taking a DNA test? Why would you get a different result from a close relative? The way DNA is inherited generation to generation is complex and some of your ancestry is erased along the way. Understanding the limits of what your own DNA can tell you about your ancestors can help you better understand why it cannot reflect your entire family tree.

I like to use a food metaphor for how you get your genome (the entirety of your DNA). Imagine if your genome is a salad. You get half your salad from mom and the other half from dad. Your sister would get a different half from mom and a different half from dad (imagine if their salad bowls are never empty and maintain the same composition just tossed around before being distributed). Your family might be a family of Italian salads, but three generations ago you had a Greek-salad ancestor. Your sister might end up with some feta in her salad from that ancestor but you might not, just by chance. So even though you have the same ancestors, your genealogy might not reflect the complete history. So it is with DNA ‘genealogy’ testing, it is limited by the mixing and loss of markers each generation. For a more detailed dive into how your genetics works check out this blogpost from evolutionary geneticist Dr. Graham Coop “Where did your genetic ancestors come from?” Graham is my genetics guru.

In reality you actually get more than half of your DNA from mom, that’s because her X chromosome that she gives you is bigger than the Y chromosome from dad (if you have that instead of another X) but also the mitochondrial DNA that mom gives you that dad doesn’t. Mitochondrial DNA is maternally inherited (from the mom side*) so we actually have a better understanding of our distant human ancestors from our maternal line because of that independent line of evidence. Mitochondria are packed into our cells, converting energy and helping to regulate cellular metabolism; they were once free-living beings that were co-opted by our distant single-celled ancestors (read about the endosymbiotic hypothesis of Dr. Lynn Margulis). We likely owe our success as multicellular organisms to this symbiosis. Mitochondria are free of the recombination we see in the nuclear DNA we discussed; they have their own chromosome that is passed down whole generation-to-generation, mother-to-mother. The way mitochondrial DNA is passed down means it doesn’t change much between generations, but it does over very long periods of time. So it isn’t that great for telling you about your great-grand mom but it is great for explaining thousands of years of human history. Listen to this TED Radio Hour with geneticist Spencer Wells looking for the “Mitochondrial Eve” and examining the spread of humanity over the last 200,000 years as told by our mitochondria.

I want to also recommend you read Carl Zimmer’s “She Has Her Mother’s Laugh: The Powers, Perversions, and Potential of Heredity,” a beautifully written book about how we understood and interpreted genetics in the past, and the powerful tools like CRISPR-Cas9 that we have at hand today. Also read Angela Saini’s excellent book ‘Superior: The Return of Race Science” to better understand how a poor understanding of genetics has been manipulated in the past and present day for nefarious purposes.

*Although there are some apparent cases of biparental inheritance

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