While searching for a kidney donor, a Boston woman stumbles upon a mystery named Karen Keegan.
When her three adult sons underwent genetic testing to determine if they were a match for a kidney donation, the tests revealed that two of them were not actually her sons.
Keegan knew she was the mother of her sons—she had conceived and given birth to them. Realizing there must have been a mistake, her doctors ran further tests,
only to uncover something even more confusing:
she was the biological aunt of her children. What is chimerism It was discovered that Keegan had another genome in some of his tissues and organs.
In other words, some of his cells had completely different genes than others. This second set of genes was from her twin sister—who was never born. This condition, where two genomes are present in an individual's body tissue, is called chimerism.
The name comes from Greek mythology, where a chimera is a combination of three different animals. People with chimerism may have two-toned skin or hair or two different colored eyes, but are thought to have no visible symptoms of the condition.
Chimerism can occur from twins in utero, from tissue or organ transplantation, or between a fetus and a pregnant woman.
How does it happen, so how exactly does it happen?
In one of the most common forms,
the cells of the mother and the fetus change in the flow of nutrients across the placenta. A mother can inherit embryonic stem cells, undifferentiated cells capable of developing into any specialized cell.
Fetal cells are initially undetectable because the mother's immune system is suppressed during pregnancy. But in some cases, cells containing fetal DNA persist in the mother's body for years or decades without being destroyed by her immune system.
In one case, a mother's liver was failing, but suddenly it started regenerating itself. Her doctors did a biopsy of her liver, and found DNA in the regenerated tissue from a pregnancy nearly 20 years earlier.
Fetal stem cells had frozen in her liver and specialized as liver cells. Karen Egan Karen Keegan, meanwhile, received her second genome before she was born.
Very early in his own mother's pregnancy with him,
Keegan had fraternal twins. Keegan's embryo absorbed some fetal stem cells from her twin embryos, 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 its body's own—so it transferred cells with other genomes.
Did not attack or destroy.
We don't know how much of his body consisted of cells with this other genome—which can vary from one organ to another, and even between tissues within an organ: some have the other genome.
Some have no cells at all, while others may have many. At least some of the egg-producing tissue in her ovary must have carried the second genome.
Each time she became pregnant,
she had no way of predicting which genome would be involved—just as two of her children ended up with genes from a woman who had never been born.
It can also happen to fathers. In 2014, after genealogy testing determined that one father was actually his child's biological uncle, researchers discovered that 10% of the father's sperm carried the other genome from the fetal twin.
Such cases challenge our perception of genetics. Although there are very few documented cases of chimerism from a fetal twin, we are all hybrids to some degree, carrying different genetic codes for our gut bacteria and even our mitochondria.
And given that 1 in 8 individual births begin as a twin pregnancy, there could be many more people with two genomes—and many more lessons to learn about the genes that make us who we are. .
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