There are only about 20 cases of twins from different fathers reported in the scientific literature.
In 2018, a woman went to the Population Genetics and Identification Laboratory at the National University of Colombia with a request: she had two twin children two years earlier and would like to confirm his paternity.
The routine test was performed and then repeated. The result was so surprising that it was necessary to be sure: the twins were children of the same mother, but from different parents.
This is an extremely unusual phenomenon known as heteropaternal superfertilization. Around 20 cases like this have been reported in scientific articles around the world.
The university’s experts knew it was possible, but they had never seen a case like this — which, naturally, sparked their scientific interest.
How was the analysis
To determine the paternity of any person, scientists at the Population Genetics and Identification Laboratory at the National University of Colombia use a technology known as “microsatellite markers“.
The technique consists, in short, of analyzing and comparing tiny fragments of DNA from the child, the mother and the supposed father.
“We took the DNA of each of them, observed 15 to 22 points, known as microsatellites, and we compare one by one“, explains Professor William Usaquén, director of the laboratory, to BBC News Mundo (the BBC’s Spanish service).
But this is not a simple process like putting DNA under a powerful microscope and looking at it.
After taking blood samples with a finger prick, scientists perform a chemical procedure to isolate DNAwhich is extremely small, and place it into specialized equipment to amplify it.
The resulting liquid is mixed with fluorescent elements to mark the 15 to 22 microsatellites that are desired to be observed.
It is then passed through another machine, capable of reading the microsatellites in each of the samples and convert them to a numeric sequence. This process is known as electrophoresis.
Finally, with the numerical sequences in hand, the researchers carry out probabilistic analysis to rule out whether or not the man is the baby’s father.
When half of the child’s genetic profile matches that of the mother and the other half matches that of the supposed father, the paternity is confirmed.
Extraordinary result
In the case of twins with different fathers discovered in 2018, scientists from the Genetics Institute of the National University of Colombia analyzed 17 microsatellites from the DNA of the mother, the two babies and the supposed father who presented himself for the test.
They concluded that the DNA of the alleged father coincided with one of the boysbut not with the other. It was an extraordinary result in every way.
“I have been director of the laboratory for 26 years and this is the first case we have witnessed”, highlights Usaquén. “And, so far, the only one.”
“We had read in other reports that these cases are observed very infrequently around the world”, says genetics specialist Andrea Casas, a researcher at the institute.
Following protocol, they repeated the test from the beginning to rule out that it was a process error or a mix-up in the samples. And they got the same result as the first time.
Why is it so uncommon
A 2014 article, published by scientists from a laboratory in Baltimore, United States, highlighted that, using a database with information on 39,000 paternity tests, they found only three cases of heteropaternal superfertilization (twins from different fathers).
Professor William Usaquén explains why this biological event occurs so infrequently.
“First of all, the mother must have two sexual partners. She also needs to have relations with both men in a short space of time.”
“Polyovulation must also occur, that is, the release of two or more eggs in the same menstrual cycle. And, finally, she needs to be fertilized both times.”
“It is a rare event, added to another rare eventanother and another rare event. Unfortunately, we don’t play the lottery”, jokes Usaquén.
It is necessary to clarify that twins from different fathers can never be identical twins, which develop from a single egg and just one sperm.
People’s intimacy
Generally, when a woman releases more than one egg and only one is fertilized, the others age and die quickly.
This is yet another reason why superfertilization is rare, as the second fertilization requires occur before the death of the excess egg.
Scientists at the Institute of Genetics calculate that both fertilizations need to occur within a period of 24 to 36 hourswhich is how long eggs remain viable after being released.
But Andrea Casas explains that “the two eggs may not necessarily come out at the same time”.
“Sometimes, a tube releases an egg and, after two or three days, releases the other,” he says. “This increases the probability of fertilization occurring at two different times.”
Of course, another reason why so few cases of twins from different fathers are known is that the immense Most people don’t take a paternity test.
Scientific literature has already indicated that, in the future, this will no longer be such an atypical phenomenon “due to the current availability of molecular methods and the popularity and increasing number of paternity tests”, as highlighted by scientists from the National University of Colombia, in their article reporting the case that they were able to prove.
Despite the possible biological interest of academics in knowing the circumstances that led to heteropaternal superfertilization, scientific ethics prevents them from questioning the intimate lives of the people who participate in the tests.
“Affiliation tests are always carried out respecting people’s integrity and privacy”, explains Usaquén.
