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Doctors have documented what they say is only the world’s second known case of semi-identical twins.

The boy and girl from Brisbane, in Australia, are now aged 4-years-old. They are identical on their mother’s side, but also share a proportion of their father’s DNA. This places them, genetically, somewhere between non-identical (fraternal) and identical twins.

The unusual discovery was made during a routine pregnancy scan at the Royal Brisbane and Women’s Hospital in 2014. This is the first time that semi-identical twins have been identified during pregnancy.

“The mother’s ultrasound at 6 weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins”, said Professor Nicholas Fisk who led the care team looking after the mother and twins. “However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

Identical twins occur in about one-third of multiple pregnancies and are known as monozygotic. This means that they were formed from a single fertilised egg, which went on to split into separate embryos. It also means that they will be the same sex and have exactly the same DNA. 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. Because these siblings all grew from separate zygotes, the babies are no more alike than any other brothers or sisters, and may be both male, both female, or one of each. All of these non-identical siblings share DNA in common, as in the case of siblings from different births.

In this case of semi-identical, or sesquizygotic, twins, the egg is thought to have been fertilised simultaneously by two sperm before it divided. If one egg is fertilised by two sperm, it results in three sets of chromosomes, rather than the standard two: one from the mother and two from the father. This phenomenon is extremely rare and doctors say that embryos such as these often do not survive.

Professor Fisk said an analysis of worldwide twin databases highlighted just how rare sesquizygotic twins were. He and his colleagues examined genetic data from 968 fraternal twins, as well as a number of large global studies, but found no other case.

The exceptional case is being reported in The New England Journal of Medicine. Twin testing from AlphaBiolabs can establish whether multiple children (such as twins, triplets, quadruplets, etc.) from the same birth are genetically identical or not. The test costs just $119 online with results in 3–5 days. For more information on Twin testing, please contact AlphaBiolabs via Live Chat or call now at 727-325-2902.