You’ve likely heard of fraternal twins. You’ve almost certainly heard of identical twins. But did you know that there is such a thing as “semi-identical twins?” There is! Though these are very, very rare.

Photo: Wikipedia

Rare or not, scientists and doctors in Australia have recently revealed that a semi-identical set of twins were born in Brisbane in 2014, only the second set of such twins ever born and the first such set ever to be identified in the womb.

In a paper just published in The New England Journal of Medicine, Dr. Nicholas Fisk at the University of New South Wales published his research and findings on the very rare genetic occurrence.

Types of Twins

Fraternal twins occur when two different ova in a mother’s womb are fertilized by two different sperm. In the case of fraternal twins, each child develops from a different egg, and therefore each shares half of their parent’s DNA.  These babies will only look as similar as any two siblings might look.

Photo: Twitter/ @DrvanTilburg

Identical twins form when a single egg is fertilized by a single sperm and then splits in two. In this case, two fetuses are formed, each of which has identical DNA and shares 100 percent of his or her DNA from both parents. Babies who are identical twins will look exactly alike, with only very small differences that spring from environmental factors or life experiences.

Both fraternal and identical twins are relatively common, with about 3 of every 100 (3%) pregnancies resulting in twins. Fraternal twins are the most common and happen most often in Africa, where one in every 20 (5%) pregnancies produces fraternal twins.

What Are “Semi-Identical Twins?”

Semi-identical twins, also called sesquizygotic twins, happen when the mother’s egg is simultaneously fertilized by two different sperm from the father.

When this happens, the egg then has three sets of chromosomes: one from the mother and two from the father (from the two different sperm). Since three sets of chromosomes are typically not compatible with life, most sesquizygotic twins are miscarried. This is why sesquizygotic twins are so rare.

Photo: Twitter/ @604Counselling

Scientists call these kinds of twins “semi-identical” because, as one researcher says, “some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion rather than 100 percent of the same paternal DNA.”

In the case of the Australian twins, doctors admit that it took quite a while to figure out what had happened because of the rarity of the event.

The First Identified Semi-Identical Twins

The first ever recorded semi-identical twins were recorded in the United States in 2007 when one of the infants was observed to have ambiguous genitalia. When doctors examined the twins further, they found that this was in part due to their unique DNA; they had identical chromosomes from their mother but only shared half of their father’s DNA.

Photo: Twitter/ @GlobalNewsLife

This was a brand new discovery at the time and led doctors to do research to see if other sets of sesquizygotic twins might have been misidentified. After searching thousands of other twin sets, they found no other semi-identical pairs.

The Brisbane Twins

All of the above demonstrates just how rare the Brisbane semi-identical twins are.

Ultrasounds of the mother in Australia originally indicated that she was having identical twins. There was one placenta, and the positioning of the amniotic sacs all indicated identical twins.

It wasn’t until further ultrasounds showed that one of the babies was male and one was female that doctors realized that the fetuses couldn’t be identical.

Doctors cared for the mother and the babies both to understand the phenomenon better and make sure the rare babies and mother were healthy.

Photo: Twitter/ @DehnTodd

Today the twins are four years old and doing well, meeting all their development milestones. Their doctor additionally reports that they are “very cute!” They reportedly look “very similar” as you might expect with so much shared DNA, but certainly not identical.

One of their doctors does note, however, that if you dressed the boy like the girl or the girl like the boy, they would look almost identical. Such is the power of clothing and appearance.

As curious as we might be to see these children, you won’t find photos of them online. The family, understandably, does not want to be identified, seeking a normal life for their very special boy and girl!