A brother and sister born in Australia in 2014 joined an exclusive club of brothers and sisters sharing an extremely rare connection: they are the second pair of "semi-identical" twins ever found.
The twins each received a DNA mix from their father, but the genes they inherited from Mom are 100% identical. Not only is there only one other case of this type known, but this couple was the first to be detected before it was born.
"The mother's ultrasound at six weeks showed a single placenta and the placement of amniotic sacs indicating that she was expecting identical twins," said Nicholas Fisk, a specialist in fetal medicine, who treated the young family while he was based at the Royal Brisbane Hospital and Women's Hospital.
"However, an ultrasound at 14 weeks showed that the twins were men and women, which is not possible for identical twins."
As a rule, twins are just two varieties. There is a non-identical "dizygotic" type, which results from two eggs each fertilized by a spermatozoon.
Then there are those who are identical, or "monozygotic", where a solitary fertilized egg divides completely into distinct individuals before settling into the planned fetal growth and development program.
Before 2007, the very idea of a third "sesquizygotic" category was more theoretical than an established fact. Then came a random discovery of twins born in the United States who turned out to be genetic chimeras.
Both of these infants possessed a mixture of cells, some with two X chromosomes and others with a Y chromosome. If one of the infants was not born intersex, it is possible that we did not let's not be more aware of their genetic secret.
Similarly, while none of the Australian twins is physiologically present as an intersexual, both possess an assortment of cells bearing pairs of XX or XY chromosomes.
The cell test taken from their respective bags of amniotic fluid also showed that, if the maternal DNA of each of them was 100% identical, only 78% of the paternal DNA matched.
A possible explanation for this assortment of genomes in one person is that the mother's eggs may be copied prematurely before being fertilized by two sperms, but have not completely separated.
There is another possibility, favored by specialists investigating the most recent case.
"It is likely that the mother's egg was fertilized simultaneously by two of the father's spermatozoa before division," says Fisk.
Like this friend who embarks on a first appointment, an additional selection of genes should sound the death knell for any incipient love story, which means that an embryo newly fertilized would not normally produce it.
"In the case of [Australian] Sesquizygous twins, the fertilized egg also seems to have divided the three sets of chromosomes into groups of cells that then split into two, thus creating twins, "says clinical geneticist Michael Gabbett of the University of Technology from Queensland, Australia.
With so few examples, it is difficult to know for sure which explanation is the most accurate or whether each group of twins has evolved slightly differently.
It is also difficult to estimate how many twins considered non-identical share the same selection of their mother's DNA.
A study of global twins databases suggests that while there are others, they are still incredibly rare examples.
"We first wondered if there were possibly other misclassified or undeclared cases, so we looked at the genetic data of 968 fraternal twins and their parents," he said. said Fisk.
"However, we have not found any other sesquizygotic twins in these data, nor any cases of semi-identical twins in the major global twin studies."
This scarcity precludes any possibility of systematic genetic screening for chimerism in twins, at least for the moment.
Advances in genetic testing and expanding databases of medical data could lead to the discovery of more semi-identical twins in the future, and could eventually help us better understand the fertilization process in more detail.
In the meantime, these twin groups can legitimately claim to be two people.
This research was published in The New England Medical Journal.