While searching for a kidney donor,
a Boston woman named Karen Keegan stumbled upon a mystery.
When her three adult sons underwent genetic testing
to determine whether they were a match for kidney donation,
the test showed that two of them weren’t actually her sons.
Keegan knew she was her sons’ mother—
she had conceived and given birth to them.
Figuring there must have been an error, her doctors pursued further testing,
only to uncover something even more confusing:
she was her children’s biological aunt.
It turned out that Keegan had a second genome in some tissues and organs.
In other words, some of her cells had a completely different set of genes
from the others.
This second set of genes belonged to her twin sister—
who had never been born.
This condition, where an individual has two genomes
present in the tissues of their body,
is called chimerism.
The name comes from Greek mythology,
where chimera is an amalgam of three different animals.
Individuals with chimerism might have two-toned skin or hair,
or two different colored eyes,
but most are believed to have no visible signs of the condition.
Chimerism can come from a twin in utero, from a tissue or organ transplant,
or happen between a fetus and a pregnant woman.
So how exactly does it happen?
In one of the most common forms,
a mother and fetus swap cells in the flow of nutrients
across the placenta.
The mother can inherit fetal stem cells,
undifferentiated cells that are able to develop into any specialized cell.
The fetal cells initially go undetected
because the mother’s immune system is suppressed during pregnancy.
But in some cases, cells with the fetus’s DNA persist
in the mother’s body for years or even decades
without being destroyed by her immune system.
In one case, a mother's liver was failing,
but suddenly started to regenerate itself.
Her doctors biopsied her liver, and found DNA in the regenerated tissue
from a pregnancy almost 20 years earlier.
The fetal stem cells had lodged in her liver and specialized as liver cells.
Karen Keegan, meanwhile, acquired her second genome before she was born.
Very early in her own mother’s pregnancy with her,
Keegan had a fraternal twin.
Keegan’s embryo absorbed some fetal stem cells from her twin’s embryo,
which did not develop to term.
By the time Keegan’s fetus developed an immune system,
it had many cells with each genome,
and the immune system recognized both genomes as her body’s own—
so it didn’t attack or destroy the cells with the second genome.
We don’t know how much of her body was composed of cells with this second genome—
that can vary from one organ to another, and even between tissues within an organ:
some might have no cells at all with the second genome,
while others might have many.
At least some of the egg-producing tissue in her ovaries
must have carried the second genome.
Each time she conceived there would be no way to predict
which genome would be involved—
which is how two of her children ended up with the genes
of a woman who had never been born.
This can also happen to fathers.
In 2014, when ancestry testing determined that a father
was actually his baby’s biological uncle,
researchers discovered that 10% of the father’s sperm
carried a second genome from an embryonic twin.
Cases like this challenge our perception of genetics.
Though there are very few documented cases of chimerism from an embryonic twin,
we’re all amalgams to some extent,
carrying around the different genetic codes of our gut bacteria
and even our mitochondria.
And given that 1 in 8 individual births started out as twin pregnancies,
there could be many more people with two genomes—
and many more lessons to learn about the genes that make us who we are.