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.
