Making sense of DNA & ancestry: 'Genetics is not genealogy'

dna

Ifound this explanatory article from the New York Times enlightening for anyone interested in tracing their lineage back through the ages through DNA.

Excerpt:

We each have two parents, the thinking goes, so therefore we inherit half of each parent’s DNA. From each grandparent we inherit precisely a quarter of our DNA, and so on by the powers of two back into the mists of time. ...

But DNA is not a liquid that can be divided down into microscopic drops. It’s a string-like molecule, arranged into 23 pairs of chromosomes, that gets passed down through the generations in a counterintuitive way.

Eggs and sperm randomly end up with one copy of each chromosome, coming either from a person’s mother or father. In the process, some DNA can shuffle from one chromosome to its partner. That means we inherit about a quarter of our DNA from each grandparent — but only on average. Any one person may inherit more DNA from one grandparent and less from another.

dna strands

Over generations, this randomness can lead to something remarkable. Look back far enough in your family tree, and you’ll encounter ancestors from whom you inherit no DNA at all.

The geneticist Graham Coop of the University of California, Davis, and his colleagues have studied how DNA disappears. If you pick one of your ancestors from 10 generations back, the odds are around 50 percent that you carry any DNA from him or her. The odds get even worse beyond that.

Even if you get no DNA from many of your ancestors, they are still your ancestors. “Genetics,” Dr. Coop has noted, “is not genealogy.”

If you use a site like 23andMe or Ancestry.com to trace your family history back hundreds of years through a DNA sample, just keep that in mind.

Images: Top, Photo by merrycallie; Center, Micah Baldwin / Creative Commons BY SA