Australian Semi-Identical Twins Are Considered Only The Second Set Ever To Be Indentified

Identified as 'semi-identical,' these incredibly rare type of twins are believed to be one of only two sets known worldwide.

Two month old twin baby sister and brother sleeping on tiny, inflatable, pink and blue swim rings
Katrina Elena / Shutterstock

Identified as 'semi-identical,' these incredibly rare type of twins are believed to be one of only two sets known worldwide.

A set of twins born in Australia in 2014 are believed to be one of only two sets identified as “semi-identical” in the world. Using genetic testing prior to the birth of the Queensland twins, the identity of this rare set of semi-identical twins was discovered.

The astounding find was recorded in the New England Journal of Medicine this week, according to U.S. Week.

Twins are usually identified as identical or fraternal. Identical, or monozygotic, twins occur when one egg splits into two embryos upon fertilization. Twins who are identical are the same sex and have the same DNA. Whereas, fraternal, or dizygotic, twins occur as a result of two separate eggs being fertilized by individual sperms. As a result of this, fraternal twins can be mixed sexes.

In the case of semi-identical, or sesquizygotic, twins, a single egg is actually fertilized by two separate sperms. Normally, this results in three sets of chromosomes that are “typically incompatible with life,” according to Dr. Michael Gabbett of the Queensland University of Technology.

However, in this instance, the egg separated completely and two viable embryos were created, leading to the creation of semi-identical twins. These twins, a boy and girl, now share the full DNA of their mother, as well as partial DNA of their father.

While experts are not entirely sure how an instance of semi-identical twins begins, Deputy Vice Chancellor for Research at the University of New South Wales, Professor Nicholas Fisk, believes that the egg is fertilized prior to it dividing and becoming two separate embryos.

Fisk first made the discovery in 2014 after discovering the twins appeared to be both identical and of different sex. Testing was done while in utero led the doctor to make the startling discovery.

“The issue here was they were of different sex, so when we did DNA studies it showed they weren’t identical, but they weren’t non-identical, either – they were really halfway in-between,” Professor Fisk told SBS News.

While the status of semi-identical will have little impact on either this set of twins or any others, Fisk points out that it is a good reminder that the medical profession should always keep an open mind.

“It reminds us that we should always keep an open mind in medicine,” he said.

“We traditionally classify twins as being identical or non-identical, and while that classification works, sometimes things don’t always appear as they seem. If there is the suspicion that something isn’t fitting into the categorization that we normally use, then we have to think outside the box.”

This extremely rare form of twins has only been identified once before. The first pair were discovered in the U.S. in 2007. This set of twins were discovered to be semi-identical when tested was done after their births.