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For first time, rare semi-identical twins identified during pregnancy

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For first time, rare semi-identical twins identified during pregnancy

They are among the rarest types of human beings — so-called “semi-identical” twins. Young Brisbane twins, a boy and a girl, are first to be identified by doctors during pregnancy and as only the second set of semi-identical, or sesquizygotic, twins in the world.

The case, the first worldwide to identify semi-identical twins on genetic testing while in the womb, has been reported in The New England Journal of Medicine (NEJM) by fetal medicine specialist, Dr Michael Gabbett.

“It is likely the mother’s egg was fertilised simultaneously by two of the father’s sperm before dividing,” said Professor Fisk, who led the fetal medicine team that cared for the mother and twins while based at Royal Brisbane and Women’s Hospital in 2014. Professor Fisk, a past President of the International Fetal Medicine and Surgery Society, worked alongside Dr Gabbett.

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  • Identical twins result when cells from a single egg fertilised by a single sperm divide into two, so identical twins are the same gender and share identical DNA. Fraternal twins occur when each twin develops from a separate egg and the egg is fertilised by its own sperm.
  • Semi-identical twins (sesquizygotic) are classed as a third type of twin, in addition to identical and fraternal twins.
  • The now four-year-old boy and girl are identical (monozygotic) on their mother’s side sharing 100 percent of their mother’s DNA, but are like siblings on their father’s side, sharing only a proportion of their father’s DNA.
  • Sesquizygotic represents a third type of ‘twinning’ between identical and fraternal (dizygotic).

“The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins. However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

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Dr Gabbett said if one egg is fertilised by two sperm it results in three sets of chromosomes, one from the mother and two from the father.

“Three sets of chromosomes are typically incompatible with life and embryos do not usually survive,” he said.

“In the case of the Brisbane sesquizygotic twins, the fertilised egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins.

“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 100 per cent of the same paternal DNA.”

Sesquizygotic twins were first reported in the US in 2007. Those twins came to doctors’ attention in infancy after one was identified with ambiguous genitalia. On investigation of mixed chromosomes, doctors found the boy and girl were identical on their mother’s side but shared around half of their paternal DNA.

Professor Fisk said an analysis of worldwide twin databases pointed to just how rare sesquizygotic twins are.

“We at first questioned whether there were perhaps other cases which had been wrongly classified or not reported, so examined genetic data from 968 fraternal twins and their parents,” he said.

“However we found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies.

“We know this is an exceptional case of semi-identical twins. While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing.”

The paper, Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning, is published in The New England Journal of Medicine on February 28.

For more details click on the link: http://dx.doi.org/10.1056/NEJMoa1701313 




Source: self

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  1. user
    Dr B. S. Bhatia March 5, 2019, 3:36 am

    ITS Amazing as this is a valuable information in the field of medical.

  2. user
    Parul Kotdawala March 4, 2019, 4:13 pm

    Years back, as a student and a resident doctor in Obstetrics & Gynec, I read about identical twins with different sexes! This was mentioned in the famed William\’s Textbook of Obstetrics. Intrigued by this, I went to the college library (B. J. Medical college, Ahmedabad), and looked up the article published in 1930s in the journal JAMA by Greulich et al. The article had pictures of 6 set of twins with different sexes, and it was concluded that some sperms have special effects and the ovum splits in to two equals rather than a big ovum and a small polar body, and each are fertilized by separate sperms! This would lead to same meternal gene component with different paternal components, which I believe this article is talking about!!
    It is amazing to note that people thought of this possibility 80 years back!
    Parul Kotdawala

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