The Population Genetics and Identification Group at the National University of Colombia uncovered the first scientific case in the country of heteropaternal superfecundation, a phenomenon where a woman gives birth to twins with different fathers. In August 2018, a woman requested DNA testing to establish the paternity of her two sons, who were two years old at the time.
A genetic analysis led by Professor William Usaquen confirmed a paternity match for one child but resulted in a paternity exclusion for the second. The findings revealed discrepancies in 14 of the 17 Short Tandem Repeat (STR) markers analyzed, a result consistent with heteropaternal superfecundation. Given the exceptional nature of the finding, the laboratory independently repeated the procedure to rule out technical errors, confirming that the children are biological twins with different fathers.
Methodology and genetic markers
The researchers obtained results by analyzing specific DNA fragments called markers. In forensic genetics, a marker is a specific location in the genome where the nucleotide sequence varies between individuals. In this case, the experts analyzed 17 genetic regions where an individual carries two variants, or alleles: one inherited from the mother and one from the father.
The laboratory first identifies the alleles that children share with their mother. The remaining variants must necessarily come from the biological father. In the first child, all paternal alleles matched the alleged father, but the second child’s profile revealed 14 inconsistencies. According to international human identification protocols, a difference in 14 markers certifies the absolute absence of a biological link, since two or three exclusions are sufficient to rule out paternity.
Mechanism and statistical probability
Heteropaternal superfecundation occurs when two eggs from the same menstrual cycle are fertilized by sperm from different men within a short time frame. This process requires polyovulation (the release of more than one egg). Scientifically, this is possible because sperm can survive in the female reproductive tract for up to five days, while an egg is only viable for 12 to 24 hours. In this case, the children’s mother confirmed to researchers that the sexual encounters occurred within a short time of each other, facilitating the synchronicity necessary for double fertilization.
In terms of probability, medical literature estimates that this phenomenon occurs in approximately one out of every 13,000 dizygotic twin conceptions where the parents request paternity testing. Other studies suggest that, in general populations, the probability is extremely low, occurring in fewer than one case per million total births, which explains why there are fewer than 20 scientifically documented cases worldwide.
Legal precedents and implications
Internationally, cases like the British twins Michelle and Lavinia Osbourne have highlighted this biological possibility. In Colombia, the finding challenges the civil system, which predicates legal paternity on the biological link. Under current legislation, the law separates child support responsibilities, linking the claimant only to the child with whom they share DNA.