When a paternity test recently revealed that a man had fathered his nephew, the internet, as you might expect, freaked out. In the ensuing frenzy, biologists offered an explanation that boggled some minds: The proud father wasn’t the father at all; he was a chimera, an individual who had absorbed the DNA of his dead twin’s fetus into his sperm.
This strange phenomenon is a scientific possibility — an inevitability, really. Many single births actually start off as multiple pregnancies; it’s just a fact of nature that fetuses sometimes die along the way. And, very rarely, dead siblings’ cells are reabsorbed by the remaining fetus.
Chimeras have existed since always, but what’s worth noting is that they are becoming increasingly common. While we may not be able to — or have any desire to — create chimeras on purpose, we can and have increased the rate of multiple pregnancies. Some fertility treatments increase the rate of multiple pregnancies and thus the chances for chimeras. If mothers continue to undergo these treatments, we will eventually see more fathers giving birth to their dead siblings’ children. This is both worthy of note and, in a larger sense, insignificant.
According to Dr. Basim Abu-Rafea, a fertility expert at the London Health Science Centre, even with the slightly inflated rates of multiple pregnancy associated with fertility treatment, chimerism happens too rarely to be worthy of much real consideration. Besides, he told Inverse, it’s been happening as long as humans have been giving birth. The rise of chimerism will mostly be attributable to chimerism being rendered identifiable by DNA testing.
Abu-Rafea says that some fertilization treatments, like ovulation induction — where a woman’s eggs are coaxed out of her ovaries off-schedule and sperm are allowed to have at it — can result in up to a 25 percent chance of multiple pregnancies. But preferable procedures, he says, put more control into the parents’ hands. During in vitro fertilization, for example, deciding the number of embryos that are implanted in the womb comes down to balancing the chances of success with the chances of multiple pregnancies.
If a sibling does die in the womb, Abu-Rafea says, the cells generally break down and are reabsorbed without consequence. “Chimerism is so rare,” he says. “It’s definitely not something I’ve come across in 14 years.”
When it comes to pregnancy — especially for those induced by fertility treatment — the chance that a kid might absorb a couple of extra cells should be the least of a parents’ concerns, says Abu-Rafea. After all, over the course of human history, multiple chimeras have been born and given birth, and nothing much has come of it.
“Life is always more powerful than anything else,” he says. “This is a good thing.”