Semi-identical twins represent a third, extremely rare form of human twinning that exists alongside the more familiar identical and fraternal types. Semi-identical twins, however, result from a completely unique biological event, placing their genetic relationship somewhere in the middle of the other two categories. This unusual twinning event leads to a genetic makeup that is only partially shared, creating a fascinating outcome for both their cellular composition and their physical appearance. The phenomenon is so rare that only a handful of confirmed cases have ever been documented globally.
Defining Sesquizygotic Twinning
The formal scientific term for semi-identical twins is “Sesquizygotic twins,” with the prefix “sesqui-” meaning one and a half. This term appropriately describes their unique genetic position, which is intermediate between identical (monozygotic) and fraternal (dizygotic) twins. Sesquizygotic twins are genetically unique because they share 100% of their mother’s genetic material but only a proportion of their father’s DNA, averaging around 78% paternal sharing in documented cases. This level of genetic overlap is significantly greater than the 50% shared by typical fraternal twins, but falls short of the near 100% shared by identical twins.
The Unique Genetic Mechanism
The formation of semi-identical twins begins with a single egg being fertilized simultaneously by two separate sperm, an event known as dispermic fertilization. This unusual process results in a zygote that initially possesses three sets of chromosomes—one from the mother and one from each of the two different sperm. Typically, an embryo with three sets of chromosomes, known as a triploid, is not viable and does not survive development.
In the rare cases of sesquizygotic twinning, the fertilized egg manages to quickly and successfully divide its three sets of chromosomes into two new cell lines. These two distinct cell lines then develop into the two separate embryos that become the twins. The resulting twins exhibit a form of chimerism or mosaicism, meaning their bodies contain cells with different genetic compositions. This means that some of the twin’s cells may carry the chromosomes from the first sperm, while other cells carry the chromosomes from the second sperm, all while sharing the mother’s full genetic contribution.
Phenotype and Physical Appearance
The question of what semi-identical twins look like is directly answered by their unique genetic makeup, resulting in an appearance that falls between the familiar twin types. Because they share all of the mother’s genes, they inherently possess a much greater similarity in many physical features compared to fraternal twins.
However, unlike identical twins, they will have noticeable differences due to the varying mix of the two paternal sperm lines in their cells. The most striking difference can be in their biological sex, as sesquizygotic twins can be a boy and a girl, which is impossible for typical identical twins. This occurs when one sperm carries an X chromosome and the other carries a Y chromosome, and the two resulting embryos receive different ratios of XX (female) and XY (male) cells. The degree of difference in their overall appearance depends on which genes were inherited from the two different paternal chromosome sets and how those genes are expressed in each twin.
Contextualizing Rarity and Discovery
Semi-identical twins are an extremely uncommon phenomenon, with only a few cases ever confirmed. The first case was reported in the United States in 2007 after doctors became suspicious of the twins’ unusual genetic markers. The unique genetic status of that initial pair was only discovered after one of the infants was found to have ambiguous genitalia, prompting a full chromosomal investigation.
The second confirmed case, which occurred in Australia in 2014, was the first to be identified while the mother was still pregnant. An early ultrasound initially suggested identical twins because they shared a placenta, but a later scan revealed the twins were a boy and a girl, leading doctors to investigate the possibility of a third twin type. This later discovery helped formally establish the classification of “Sesquizygotic” as a unique category of twinning. Genetic testing is typically required to confirm this diagnosis, as standard prenatal screening often classifies them incorrectly due to their shared placenta.