What can DNA tests really tell us about our ancestry? - Prosanta Chakrabarty

Two sisters take the same DNA test.

The results show that one sister is 10% French, the other 0%.

Both sisters share the same two parents,

and therefore the same set of ancestors.

So how can one be 10% more French than the other?

Tests like these rely on our DNA to answer questions about our ancestry,

but our DNA actually can’t tell us everything

about who we are or where we’re from.

DNA tests are great at answering some questions,

like who your parents are, but can provide baffling results to others,

like whether you have ancestors from a particular region.

To understand why, it helps to know where our DNA comes from in the first place.

Each person’s DNA consists of about 6 billion base pairs

stored in 23 pairs of chromosomes— 46 total.

That may seem like a dizzying amount of information,

but 99% of our genome is shared among all humans.

The remaining 1% contains everything distinct about an individual’s ancestry.

Commercial DNA tests utilize less than 1% of that 1%.

One chromosome in each pair comes from each parent.

These halves join at conception: when a sperm and egg,

each with only 23 chromosomes, combine.

The story of our ancestry becomes muddled before conception.

That’s because the 23 chromosomes in a sperm or egg

aren’t identical to the chromosomes of every other cell in the body.

As they go from a cell with 46 chromosomes to a sex cell with only 23,

the chromosomes within each pair swap some sections.

This process is called recombination, and it means that every sperm or egg

contains single chromosomes that are a unique mash up of each pair.

Recombination occurs uniquely in each sex cell—

making two sisters’ chromosomes different not only from their parents’,

but from each other’s.

Recombination happens before conception,

so you get exactly half of your DNA from each parent,

but going further back things get more complicated.

Without recombination, you would get 1/4 from each grandparent,

1/8 from each great-grandparent, and so on,

but because recombination happens every generation, those numbers vary.

The more generations removed an ancestor is,

the more likely they won’t be represented in your DNA at all.

For example, without recombination,

just 1/64 of your DNA would come from each ancestor six generations back.

Because of recombination, that number can be higher,

though we don’t know for sure how high— or it can as low as 0.

So one sister isn’t more French

in the sense of having more ancestors from France.

Instead, the French ancestors are simply more represented in her DNA.

But the story doesn’t end there.

Tests don’t trace the DNA of the sisters' actual French ancestors—

we don’t have access to the genomes of deceased individuals

from previous generations.

Instead, these results are based on a comparison

to the DNA of people living in France today.

The tests look for genetic markers, or combinations of genetic markers.

These markers are short sequences that appear in specific places.

The sister deemed "more French" shares genetic markers

with people currently living in France.

The assumption is that these shared markers indicate ancestors

from the same place: France.

It’s important to note that results are based on people

who’ve had their genomes sequenced—

80-90% of which are of European descent.

Many indigenous peoples are barely represented, if at all.

The test won’t reveal heritage from people not represented in the database,

and shouldn’t be used to prove race or ethnicity.

And as more people get sequenced, your results might change.

Looking further back, you may get a result like 2% Neanderthal.

Though Neanderthals were a separate species from humans,

that 2% doesn’t come out of the 99% of our genome shared among all humans,

but the 1% that varies.

That’s because about 40,000 years ago,

certain human populations interbred with Neanderthals,

meaning some people alive today have Neanderthal ancestors.

Many Neanderthal ancestors, in fact:

there are so many generations in 40,000 years

that a single Neanderthal’s genetic contribution would be untraceable.

You can be both 100% French and 2% Neanderthal—

though both come from the 1% of DNA that makes us different,

they’re accounting for different things.

Looking for traces of our ancestry in our DNA gets complicated very quickly.

Both the way we inherit DNA and the information available for testing

makes it difficult to say certain things with 100% certainty.