Before Arguing About DNA Tests, Learn the Science Behind Them

["Dave Farber" <farber () gmail com>]

Begin forwarded message:

> From: Dewayne Hendricks <dewayne () warpspeed com>
> Date: October 19, 2018 at 7:21:44 PM GMT+9
> To: Multiple recipients of Dewayne-Net <dewayne-net () warpspeed com>
> Subject: [Dewayne-Net] Before Arguing About DNA Tests, Learn the Science Behind Them
> Reply-To: dewayne-net () warpspeed com
>
> Before Arguing About DNA Tests, Learn the Science Behind Them
> Our genetic code cannot be treated as a matter of simple fractions.
> By Carl Zimmer
> Oct 18 2018
> <https://www.nytimes.com/2018/10/18/opinion/sunday/dna-elizabeth-warren.html>
>
> People have always told stories about their ancestral origins. But now millions of people are looking at their DNA to 
> see if those stories hold up. While genetic tests can indeed reveal some secrets about our family past, we can also 
> jump to the wrong conclusions from their results. 
>
> The reception of Senator Elizabeth Warren’s DNA results is a textbook case in this confusion.
>
> On Monday morning, Senator Warren released an analysis on her DNA showing that six to 10 generations back she had a 
> Native American ancestor. Within hours, Michael Ahrens, an official at the Republican National Committee, dismissed 
> the results in a tweet:
>
> “So Elizabeth Warren is possibly 1/1024 (0.09 percent) Native American. Scientists say the average European-American 
> is 0.18 percent Native American. That’d make Warren even less Native American than the average European-American.” 
>
> By Monday afternoon, James Freeman, an assistant editor of The Wall Street Journal’s editorial page, had fleshed out 
> Mr. Ahrens’s arithmetic. The DNA analysis, he wrote, “suggests that the senator is somewhere between 1/64th and 
> 1/1024th Native American.” He added: “Her genetic makeup is perhaps similar to that of the average white person in 
> the U.S.”
>
> These numbers then began ricochet around social media. They carried a clear implication: that Elizabeth Warren was no 
> different in her Native American ancestry than a great many other white Americans.
>
> Both Mr. Ahrens and Mr. Freeman cited a 2014 New York Times article as evidence for their claims. I wrote that 
> article. So let me just say this: They’re wrong.
>
> They both mistakenly treat DNA as a matter of simple fractions. 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. This is how they came up with the 1/1024th figure — two 
> parents, each with two parents, going back 10 generations.
>
> This misguided way of thinking has a history that extends far beyond the discovery of DNA. For centuries, people 
> thought of ancestry in terms of blood, and fractions of it. People were pure-blooded or half-blooded. When the United 
> States government set up rules for deciding who could be members of Native American tribes, it called the system 
> “blood quantum.”
>
> Slavery, too, led to an obsession with increasingly tiny fractions of ancestral blood, reaching the absurd extreme of 
> the “one drop” rule. A single black ancestor — no matter how far back in the family tree, no matter how tiny the 
> mythical drop of blood he or she contributed — was enough to make a person black.
>
> Blood still soaks our language about heredity. Reporting Monday on Senator Warren’s test, The Atlanta Journal 
> Constitution wrote that “she may have Cherokee blood.”
>
> 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.
>
> 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.”
>
> To understand Senator Warren’s ancestry, the geneticist Carlos Bustamante of Stanford University examined the 
> sampling of DNA she inherited from some of her ancestors. He used software he and his colleagues have developed to 
> compare stretches of DNA in one person to those in different populations.
>
> [snip]
>
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> Twitter: https://twitter.com/wa8dzp



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