The unusual case of the Colombian woman who became pregnant by two men at the same time (and what the scientists who studied the case discovered)

Imagine discovering that your twins are the children of two different men, and that this is not the plot of a novel, but a real biological phenomenon. In Colombia, a woman went to a genetics laboratory to confirm the paternity of her twin sons, and the scientists encountered something so rare that, in 26 years of experience, the laboratory director had never seen it before: the boys had the same mother, but each had a different father. Thus, heteropaternal superfecundation was discovered—an event so improbable that, in a database of 39,000 paternity tests in the United States, only three cases were found. Professor William Usaquén summed it up humorously: “It's a rare event on top of another rare event, plus another rare event, plus another rare event. Too bad we don't play the lottery.” For this to happen, several factors must come together: the woman must have two sexual partners within a short period, ovulate twice in the same cycle, and both eggs must be fertilized within just 24 to 36 hours, which is how long eggs survive after being released. The geneticists who studied the case used a technique called “microsatellite markers”: they analyzed between 15 and 22 tiny DNA fragments from the mother, the children, and the alleged father. They were utterly surprised when they found that the man's DNA matched only one of the twins. They repeated the process, ruling out errors. The result: The extraordinary was real. Most of the time, when a woman releases more than one egg and only one is fertilized, the others die quickly, which is why superfertilization is so rare: the second egg must be fertilized before it dies. Furthermore, as geneticist Andrea Casas explained, eggs can be released even days apart, which slightly widens the window for this phenomenon to occur. Here's a fact that could change our perception: fraternal twins with different fathers can never be identical twins, because identical twins originate from a single egg and a single sperm cell. Now, although science is aware of this, most people never find out because almost no one gets a paternity test for fraternal twin. However, experts believe that as these tests become more common, we will start to discover more cases, and what seems incredible today could become less rare than we think. There is an important ethical consideration: scientists never ask about the private lives of those who come to the laboratory, because their priority is always the privacy and integrity of individuals. If you grew up thinking that only movies feature impossible stories, biology can be even more outlandish. When the exception breaks the rule, it forces us to rethink what we take for granted about parenthood and the limits of nature. If this case makes you see genetics in a different light, you can mark it with I'm In in Lara Notes: it's not a like; it's a gesture that says, “This concerns me; now I know about it.” And if this story becomes a dinner-table topic, in Lara Notes you can mark it as Shared Offline: that way, you let that person know the conversation was special, and it's saved. This journey through improbable genetics is brought to you by BBC News Mundo and has saved you 3 minutes of reading time.