Australia: Second case of semi-identical twins, a rare phenomenon, found
What's the story
In Australia, a pair of twins have been identified as semi-identical or sesquizygotic twins, which is a rare phenomenon, in which two sperms fertilize the same egg.
The twins were born in 2014 and are the first ones to be identified while they were in their mother's womb.
Notably, they are the second known case of sesquizygotic twins.
Here's more on this.
Twins
Let's first understand about the two general sets of twins
Usually, there are two sets of twins- identical or monozygotic and fraternal or dizygotic.
Identical twins are created from a single fertilized egg that later splits in two, creating two identical boys or two girls. Their DNAs match 100%.
On the other hand, fraternal twins happen when two eggs get fertilized by two sperms, creating two genetically unique siblings. Their DNAs match 50%.
Details
Twins are identical from mother's side but siblings from father's
Coming back to the case, it was published in the The New England Journal of Medicine this week.
The pair, a boy and a girl, took birth at Royal Brisbane and Women's Hospital.
Both are identical (monozygotic) on their 28-year-old mother's side and share 100% of her DNA.
However, from their father's side, the two, like siblings, share only a proportion of his DNA.
Ultrasound
Initially doctors believed the twins were identical
Professor Nicholas Fisk, at the University of New South Wales, told media, "It is likely that the mother's egg was fertilized simultaneously by two of the father's sperm before splitting."
When the twins' mother was six weeks pregnant, her ultrasound displayed a set of twins sharing one placenta, which led doctors to believe they are identical twins.
However, the 14-week ultrasound revealed different genders.
Risk
One egg-two sperms produced embryos usually don't survive
Fisk explained that one egg getting fertilized by two sperms shouldn't happen because once the sperm enters the egg, the egg locks itself to prevent thousands of other sperms to enter.
If two enters, three chromosomes come out, one from the mother and two from the father and these embryos usually don't survive, explained Michael Gabbett, clinical geneticist at Queensland University of Technology.
First case
First case was identified when one twin had ambiguous genitalia
Gabbett further said that, however, in this case, "the fertilized egg appears to have equally divided-up the three sets of chromosomes into groups of cells, which then split into two, creating the twins."
The first-case of sesquizygotic twins was seen in the US in 2007 when one of the twins was identified with ambiguous genitalia, a birth-defect in which the newborn's gender is unclear.