In 2018, a woman came to the Population Genetics and Identification Laboratory of the National University of Colombia with a request: she had had twin sons two years ago and wanted to confirm their paternity.
They did the routine test and then did it again. The result was so surprising that they wanted to be sure: the twins were children of the same mother but different fathers.
This is an extremely rare phenomenon known as heteropaternal superfecundation. About twenty cases of this have been reported in scientific articles worldwide.
Although they knew that something like this was possible, the experts at the National University had never encountered a case firsthand.
And, of course, it sparked his scientific interest.
How they found out
To determine the paternity of any person, scientists at the Laboratory of Population Genetics and Identification of the National University of Colombia use a technology called “microsatellite markers.”
This consists, broadly speaking, of analyzing very small fragments of DNA from the child, the mother, and the supposed father, and comparing them.
“We take the DNA of each of them, we look at between 15 and 22 points, which are called microsatellites, and we compare them one by one,” explains Professor William Usaquén, director of the laboratory, to BBC Mundo.
But the process is not as easy as putting the DNA under a powerful microscope and observing it.
After taking blood samples with a finger prick, scientists perform a chemical procedure to separate DNA from non-DNA.
They then take the DNA, which is extremely small, and run it through specialized equipment to amplify it.
The liquid that results from this is mixed with fluorescent elements to mark 15 to 22 points (microsatellites) that they want to observe, and they pass it through another machine, capable of looking at the microsatellites in each of the samples and converting them into a numerical sequence. This process is called electrophoresis.
Finally, with the numerical sequences in hand, the researchers do a probabilistic analysis to rule out whether or not the man is the father of the baby.
When half of the child’s genetic profile matches that of the mother and the other half matches that of the alleged father, paternity is confirmed.
An extraordinary result
In the case of the twins from different fathers who were discovered in 2018, scientists at the Institute of Genetics of the National University of Colombia analyzed 17 microsatellites in the DNA of the mother, the two babies and the alleged father who took the test.
And they found that the alleged father’s DNA matched that of one of the children, but not the other.
It was a clearly extraordinary result.
“I have been director of the laboratory for 26 years, and it is the first case we have seen, and so far the only one,” says William Usaquén.
“We had heard from other reports that these cases were observed very rarely in the world,” says Andrea Casas, a genetics expert and researcher at the Institute of Genetics.
As established by the protocol, they repeated the test from the beginning to rule out an error in the process or a mix-up of the samples.
The result was the same as the first time.
Why is it so uncommon?
A 2014 article published by scientists from a laboratory in Baltimore, United States, pointed out that in a pool of data with information on 39,000 paternity tests, only three cases of heteropaternal superfertilization (twins from different fathers) were found.
Professor William Usaquén explains why it is a biological event that occurs so infrequently:
“First, the woman must have two sexual partners. Second, she must have relations with both men in a short period of time. In addition, there must have been polyovulation (that is, the release of two or more eggs in the same menstrual cycle). And, finally, she must be fertilized both times.”
“It’s a rare event added to another rare event, plus another rare event, plus another rare event. Unfortunately, we don’t bet on the lottery,” Usaquén jokes.
It should be noted that, in no case, twins from a different father can be identical twins, since the latter develop from a single egg and sperm.
The privacy of people
Typically, in cases where the woman releases more than one egg, when only one is fertilized, the other or others age and die quickly.
This is also why superfertilization is rare: because the second fertilization occurs before the unfertilized egg dies.
According to scientists at the Institute of Genetics, both fertilizations have to occur within a period of between 24 and 36 hours, which is the time that the eggs remain viable after being released.
However, explains Andrea Casas, “the eggs may not necessarily come out at the same time.”
“Sometimes, one tube releases an egg, and after two or three days, it releases the other.” […]”, which increases the probability that fertilizations occur at two different times, he adds.
Of course, another reason why so few cases of twins from different fathers are known is that the vast majority of people do not take a paternity test.
In fact, it has been suggested in the scientific literature that in the future this will no longer be such an atypical phenomenon “thanks to the exact availability of molecular methods and the popularity and growing number of paternity tests,” the scientists from the Institute of Genetics point out in the article they published reporting the case they were able to verify.
Despite the biological interest that academics may have in knowing the circumstances in which heteropaternal superfertilization occurred, research ethics prevents them from asking about the intimate lives of the people who take the test.
“Affiliation tests are always done taking into account the integrity and privacy of the people,” explains Usaquén.
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