So, you’ve heard of identical twins, right? You may even personally know a set — or are at least familiar with a famous set, such as Tia and Tamera Mowry or Dylan and Cole Sprouse. A type of twin you may be much less familiar with, though, is semi-identical twins. But with twin births on the rise due to advances in fertility treatments, this rare type of twinning could become less of a mystery in the near future. So, let’s go ahead and get you acquainted.
After all, if you’re trying to conceive (either naturally or via fertility treatments), there’s a chance you’ll wind up a mother of twins. Here’s everything to know about semi-identical twins.
What are the “main” types of twins?
In terms of twins, most fall under two primary categories: fraternal or dizygotic (the most common type), and identical or monozygotic. Knowing how these types of twins are formed serves as an important basis for the understanding of semi-identical twins.
With monozygotic twins, a single egg is fertilized by two different sperm. After fertilization, that egg splits. The result? Babies that are born with identical DNA and, therefore, identical genetic traits like sex and eye color. With dizygotic twins, two separate sperm fertilize two separate eggs. In this case, the twins only share 50 percent of their DNA, making them no more alike than non-twin siblings.
So, what is a semi-identical twin?
In a report published in the 2007 edition of the Journal of Human Genetics (per BBC), the term semi-identical — or sesquizygotic — first surfaced as a descriptor for a type of twins. Somewhere between fraternal and identical, these twins share 100 percent of DNA on their mother’s side but only about 50 percent on their father’s side. Scientists suggest this happens when two sperm fertilize a single egg. This results in what is known as a triploid, or a cell containing three sets of chromosomes. The egg then splits, resulting in three sets of chromosomes but only two fetuses.
How rare are semi-identical twins?
Very, very rare. In fact, only the second confirmed case of semi-identical twins was reported in 2019 by the New England Journal of Medicine. Normally, embryos with three sets of chromosomes do not survive and result in a miscarriage.
But, as we now know, it does happen. In the case of the twins reported in 2019 (who were born in 2014), they were found to be “100 percent identical on their mother’s side and 78 percent identical on their father’s side.” This averages out to them being 89 percent identical. “In the case of the Brisbane sesquizygotic twins, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins,” Dr. Michael Gabbett, clinical geneticist and diagnostic genomics course coordinator at Queensland University, explained in a press release.
Semi-identical twins are so rare that some researchers don’t expect another set will ever be confirmed. However, it’s possible there exist cases of semi-identical twins in the world that just haven’t been identified yet. Unless the twins are different genders, the assumption at birth may just be that they’re identical.
What’s the difference between semi-identical and half-identical?
Okay, so we have a basic grasp of what happens to create semi-identical twins: a single egg splits after being fertilized by two sperm. Well, with half-identical twins, a single egg splits and then each of its halves meets and gets fertilized by sperm. However, there have yet to be any confirmed cases of half-identical twins. There aren’t even any definitive tests to confirm half-identical (also known as polar body) twins if they are suspected. To that end, some scientists doubt they actually exist.
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