Super Rare 'Semi-Identical' Twins Discovered In Australia — WHOA

A chart explaining how different types of twins are conceived

Doctors in Australia say they have identified a pair of rare "semi-identical" twins. They are the first to have been observed in utero (in the womb).

Once an egg is fertilized, it becomes impenetrable to other sperm, so Gabbett believes that the two sperm must have arrived at the egg at the same time.

He was tipped off when he spotted that the twins were different sexes, yet seemed to have nearly identical DNA.

Identical twins are formed when a single fertilised egg splits into two, meaning their DNA is exactly the same.

"Now, that shouldn't happen - when a sperm enters an egg, the egg locks down to prevent one of the millions of other sperm getting in there as well". This is similar to siblings who are not twins.

Identical twins do not run in families.

Identical twins (also known as monozygotic - originating from one zygote) are less common.

The rate of identical twins is consistent across the globe at about four in 1,000 births. As a result, they received exactly the same set of DNA from their mother, but different from their father as the eggs were fertilised by different sperm.

But now we know the Queensland siblings are the second set of twins to fall into this mysterious and fascinating category.

What are semi-identical twins?

Non-identical, or dizygotic, twins occur when two separate eggs are fertilised, each by a different sperm, and develop in the womb at the same time.

The only other reported case of sesquizygotic twins occurred in the United States in 2007, when the boy and girl were infants.

However, when another ultrasound was conducted 14 weeks later, it showed she was going to have a male and a female child - which is not possible for identical twins.

Doctors initially thought that twins were identical following an ultrasound scan where they were sharing the same placenta. They were an anomaly.

Further genetic testing of the twins' amniotic fluid revealed that they were maternally identical but shared only part of their father's DNA, the report concluded.

But they share only a proportion of their father's DNA - placing them, genetically, somewhere between fraternal and identical twins.

When this doesn't go to plan, a baby may get too many or too few chromosomes, resulting in genetic disorders like Down Syndrome (an extra copy of Chromosome 21).

But in the semi-identical twins, one set of chromosomes came from the egg, and the second set was made up of chromosomes from two separate sperm, Gabbett told Live Science. This may remain a scientific mystery.

The sister and brother share identical DNA from their mother, but not from their father, New Scientist reported. Three years later, doctors discovered that the same twin had a condition called gonadal dysgenesis, a condition affecting the development of the body's sex organs.

A search by researchers through huge databases of twins across the globe failed to find more semi-identical twins, suggesting this type of twinning is very rare.

Discoveries like this that teach us just how much there is still to learn about biology and health, and how fascinating the world of reproduction and pregnancy is.

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