In 2014 an Australian woman found out that she was having twins – not by itself news that would make headlines. But when the expectant mother checked in with her doctor, the medical professional was left stunned by what he found. You see, the two fetuses fell under neither the identical nor the fraternal category; instead, they made up an incredibly rare set of semi-identical twins.
Nevertheless, for the Australian woman at the center of this story, her path to motherhood began in typical fashion. Indeed, in February 2019 the BBC reported doctors at the Royal Brisbane & Women’s Hospital as saying that she had become pregnant naturally. And as with most moms-to-be, the woman in question underwent the usual pregnancy scans in order to ensure that the fetuses had begun to grow healthily.
In most cases, medical professionals employ ultrasound imaging to examine the womb and identify any potential birth defects or similar issues that could be present in a fetus. Ultimately, then, it was during this routine procedure that Professor Nicholas Fisk noticed something unusual about the Australian mom’s offspring.
At first, Fisk said, the woman’s pregnancy seemed to be charting a very traditional course. “The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins,” he told the BBC. In time, however, Fisk noticed a single detail on the screen that proved the fetuses couldn’t actually be identical.
Parents of multiples will almost always find that they’re expecting one of two different types of twins: monozygotic or dizygotic. In any pregnancy, you see, sperm fertilizes at least one egg in order to create the zygote cell. Sometimes, though, this cell will split into two embryos, and because the embryos come from the same cell, they have the exact same DNA.
As such, monozygotic twins tend to appear as identical as their DNA and are both of the same gender. Despite the similarities in the twins’ appearances, however, they will have different fingerprints. Why? Well, as each baby grows in the womb, they feel around their environment and gather unique markings that differentiate the grooves on their digits.
And across the planet, approximately 0.3 percent of births see a set of identical twins into the world. What’s more, the likelihood of a woman having lookalike babies is the same everywhere on the planet, although this isn’t the case when it comes to dizygotic – or fraternal – twins.
Fraternal twins are also known as “non-identical twins” – suggesting, of course, that they do not end up looking the same. That’s largely because the two babies grow from separate eggs fertilized by different sperm cells. And as such, each twin possesses distinct DNA; the pair just happen to share the womb and arrive at the same time.
So, while fraternal twins can still come out as spitting images of each other – indeed, this can happen with siblings of any kind – their unique DNA means that they can also look totally different, too. And then there’s the possibility of boy-girl dizygotic twins as well, which isn’t the case with identical pairs.
Plus, instances of fraternal twins don’t have a universally uniform distribution; instead, different areas of the planet inexplicably have more dizygotic twins than others. Japan has about six sets for every 1,000 deliveries, for example, while areas of India have 15. And in some spots in Central Africa, more than 20 births per 1,000 bring two bouncing – but not genetically identical – babies into the world.
It’s worth noting, too, that fertility treatments and medications have boosted the number of fraternal twins. Typically, these drugs cause mothers to hyperovulate, which means their bodies emit numerous eggs at once. It’s the perfect scenario, then, for more than one embryo to form. And because of this phenomenon, the numbers of multiple births – from twins to triplets to quadruplets and beyond – have spiked in the U.S.
With all of this information in mind, then, it becomes easier to understand why Fisk found himself confused when he performed an ultrasound for the Australian mom-to-be in 2014. At first, the doctor saw a pair of amniotic sacs that were each on the same placenta – usually a surefire sign that the mom would have identical twins.
After that, Fisk told the mom that she’d have to come back for another check-up to determine the sex of her fetuses – although it may have appeared safe to assume that they’d either be two boys or two girls. But at the mom’s 14-week ultrasound, Fisk found himself facing the unexpected. Yes, the mom had one boy and one girl on the way – even though the fetuses were growing in a monozygotic manner.
And according to a March 2019 report by The New York Times, Fisk thought to himself at this moment, “It doesn’t add up.” Eventually, though, he realized that he had overlooked a third twin category – one so rare that there’s only one other recorded instance.
Specifically, Fisk discovered that the twins were semi-identical – also known as sesquizygotic. Therefore, the fetuses had formed after one egg in their mother’s womb had been fertilized by separate sperm. And as a consequence, while the twins each shared differing amounts of their father’s DNA, they both had 100 percent of their mom’s.
“Broadly, they are about three-fourths identical,” Fisk told The New York Times before adding that such a situation is “almost as rare as hen’s teeth.” In theory, you see, such a conception shouldn’t be feasible, especially considering the two ways in which twins typically form. In both scenarios, one sperm plus one egg equals one baby.
“When a sperm enters an egg, the membrane locks down to stop any other sperm from getting in,” Fisk explained. “Even if another sperm got into an egg, you would end up with three sets of chromosomes.” And owing to this excess of genetic material, Fisk added, the resulting fetus ultimately wouldn’t survive.
But semi-identical twins nonetheless occur because they each somehow receive only the amount of genetic information that they need. “Some of the cells contain the chromosomes from the first sperm, while the remaining cells contain chromosomes from the second sperm,” Fisk’s colleague, Dr. Michael Gabbett, explained to CNN in March 2019.
As such, Gabbett said, this resulted “in the twins sharing only a proportion rather than 100 percent of the same paternal DNA.” And, in fact, staff at the Royal Brisbane & Women’s Hospital embarked on a painstaking examination of each twin’s DNA, comparing all of the female fetus’ chromosomes to her brother’s.
In some stretches of the twins’ DNA, they shared the exact same genetic information; in other parts, though, they were different. Ultimately, then, this amounted to the fetuses sharing approximately 89 percent of their DNA. And the process also made Fisk the first doctor to identify semi-identical twins during their mother’s pregnancy.
The only other recorded case of semi-identical twins occurred in 2007. In that situation, however, the female twin required medical care because she had what is termed “ambiguous genitalia.” And when doctors examined the female fetus’ genetic code, they realized that she and her twin had identical DNA from their mother but only about 50 percent of the same code from their father.
Neither of the Australian twins had to face this hurdle, though. Indeed, they came into the world healthy and, as Fisk told The New York Times, they continue to thrive. “At age four, they are doing well and meeting their developmental goals,” he said.
Still, once Fisk and researchers from two Australian universities realized that they had this rare set of semi-identical twins on their hands, they couldn’t help but wonder how many other sesquizygotic babies existed out there. Sure, there was the 2007 case, but they thought that other twins may have been misidentified or, perhaps, that their varied DNA may not have been chronicled.
So, to prove the rarity of the semi-identical twins, Fisk and his colleagues from the University of New South Wales and the Queensland University of Technology went on to examine the DNA of close to 1,000 sets of fraternal twins. And Fisk went on to describe the monumental task to CNN, saying, “We needed to confirm the results with really exhaustive studies in multiple labs in multiple states and countries and with tests involving millions of genetic variants.”
Then, with the investigation finally complete, the team could be sure of their initial notion: the semi-identical twins delivered in Australia were indeed extremely rare. Nevertheless, before the researchers shared their findings through The New England Journal of Medicine, they had had to be thorough. Fisk later acknowledged to CNN, “We needed to convince rightly skeptical reviewers that this [data] really was robust.”
And as it turned out, the group’s work piqued the interest of other sets of twins. Indeed, Fisk told The New York Times that he had received numerous emails from those who thought that they and their twin may have gone through life without knowing they were more than just a fraternal pair.
But, of course, even with Fisk and his team’s findings, many questions remain about semi-identical twins. In fact, Dr. Mindy Christianson of the John Hopkins School of Medicine said that the research may have only scratched the surface of what can happen when twins are conceived.
“The recent case was detected because the twins were male and female,” Christianson told The New York Times. “There may be other same-sex twins out there that are also semi-identical.” Nevertheless, Fisk and his colleagues did consider this possibility, and yet their examinations of almost 1,000 fraternal twins didn’t uncover any semi-identical pairs among the study group.
For Fisk, though, the loose ends that remain are all part of the fun of being a researcher of twins, even if he acknowledges that there’s still work to be done on the subject. For instance, although identical twins seem to be a clear-cut phenomenon, Fisk believes that there could be more to be learned on understanding how they form.
Yet while the semi-identical twins at the center of this breakthrough had practically been proved to be medical miracles, their family seemingly don’t want the pair to become notorious. In any case, the twins’ identities have not been released to the media – although there has since been an update on how they’re thriving. In February 2019 Gabbett told ABC, “[The four-year-olds] look very similar. They’re very cute, and they’re doing very well.”
And, of course, the Australian babies haven’t been the first pair of twins to cause a stir, though most others grab attention for their physical characteristics rather than their genetic information. In 2017, for example, images of twin baby girls Kalani and Jarani Dean caused waves because the sisters looked so different from each other.
What’s more, the babies’ mother, Whitney Meyer, realized the clear dissimilarity in appearance as soon as Kalani came into the world on April 23, 2016. As Meyer later told Today, “Kalani was as white as can be. I was just in denial because, you know, the odds of this?” In essence, the new mom was shocked because, while she is white, her boyfriend, Tomas Dean, is black.
Yet Dean didn’t register the same level of surprise until he saw Kalani’s twin sister, Jarani. “I was like, ‘Yeah, [Kalani’s] a little light,’ but I thought maybe babies are that way when they’re first born,” he said. “But then, a couple of minutes later, her sister came out a little darker.”
And Meyer was right to wonder how often such a situation occurs, since most biracial siblings – especially twins – are more similar in appearance than Kalani and Jarani. But then again scientists still haven’t yet figured out all of the ways in which genes dictate a baby’s looks. In short, then, there’s no answer yet.
Even so, in 2017 University of California, San Francisco geneticist Dr. Bryce Mendelsohn explained to Today just what may have happened in the case of the twin girls. “The physical traits you can see in a person are just a very small sliver of the genetic diversity across human populations,” he said.
“A lot of times, we only focus on the things our eyes can see. But what we see is a tiny tip of the iceberg of the actual genetic diversity in everyone,” Mendelsohn continued. “When you flip a coin eight times, sometimes it’s going to be heads all eight times. And it’s kind of like that when you have a bunch of genes. They’re all randomly shuffling, and you can get all kinds of outcomes.”
Still, this scientific explanation didn’t necessarily mean that the rest of the world would understand how Kalani and Jarani could look so different and yet still be twins. To help bridge the gap, then, Meyer bought the girls a matching wardrobe so that they looked alike in that way. “I have to dress them the same because nobody believes that they are twins. I mean, nobody,” she said.
But the doubters did little to discourage Meyer, who saw her biracial daughters as a powerful symbol that we should all “love everyone equal.” The mom added of her twins, “You can’t look at one and not love them both. They’re the same girl – just different colors.”
Meanwhile, while Dean could understand why people had taken an interest in his daughters, he also pointed out that their physical features make up only a fraction of who they are. “I hope that a lot of people can see that color really isn’t a big thing,” the dad remarked to Today. “What’s important is love. Mysterious things can happen, and life is a blessing.”
And two years after Kalani and Jarani’s story appeared on the Today website, the girls continue to touch hearts – including their mother’s. In fact, on January 28, 2019, Meyer revealed on Facebook that her daughters’ tale had been re-shared in commemoration of Martin Luther King, Jr. Day. “People really did have the pleasure [of] reading our story and fell in love with them… I’m a proud mother, and their daddy is proud, too,” she wrote.