A semi identical twin test can determine whether or not your twins are semi identical, or half-identical. This form of twin is extremely rare and has only ever been identified twice.

This latest category of twin adds to the better-known classifications of identical and non-identical (fraternal twins).

  • Identical twins are formed from a single fertilised egg, which went on to split into separate embryos. This means that the siblings share 100% of their DNA and they are known as monozygotic. The twins will either be two identical boys or two identical girls.
  • Non-identical (or fraternal) twins are the result of separate fertilised eggs and are known as dizygotic, which means they have two zygotes. This produces two genetically unique siblings, which are no more alike than any other brother or sister, and may be both male, both female, or one of each. They share 50% of their DNA as in the case of brothers and sisters.  
  • Semi identical twins are the result of a single egg being fertilized by two sperm –something that shouldn’t happen as usually once a sperm enters the egg, the egg locks down in order to prevent another sperm from entering. This results in three sets of chromosomes, rather than the standard two: one from the mother and two from the father. These twins are known as sesquizygotic.

The first documented case of semi identical twins was in the USA in 2007. The second case was identified in Australia. Professor Nicholas Fisk, who led the team that cared for the mother and twins at the Royal Brisbane and Women’s Hospital in 2014, said the discovery was made through a routine pregnancy scan.

“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 Fisk. “However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

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

“While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing”, he said.

Semi identical twin test from AlphaBiolabs

Semi identical twin testing is possible from AlphaBiolabs. Its zygosity twin test, which can determine whether your twins are identical or non-identical, can be adapted to test for semi identical twins too. A testing kit can be sent direct to your home with full instructions on how to collect your samples.

Zygosity twin testing involves analysing tiny amounts of DNA from inside each sibling’s mouth. All you need to do is rub a cotton bud on the inside of each twins’ cheek. When these samples are returned to our laboratory, our scientists examine specific markers present in repeat sections of DNA. The DNA seen at each of these 42 Short Tandem Repeat (STR) loci is compared between the tested individuals; identical twins will share the same DNA profile whereas non-identical siblings will have different DNA profiles.

In the case of semi identical twin testing, your zygosity test can be upgraded to include a DNA analysis of the twins as well as both parents. It is only by analysing all four samples of DNA that a diagnosis of semi identical twins can be made. Should the initial zygosity test prove your twins to be identical, you can choose to upgrade to the semi identical test.

For more information on Zygosity twin testing, please contact AlphaBiolabs’ Customer Service team on 0333 600 1300, email: or visit our website at

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