A brother and sister born in Australia in 2014 have joined an exclusive club of brothers who share an extremely rare bond – they are the second pair of "semi-identical" twins ever found.
The twins received a mixture of DNA from dad, but the genes that they inherited from mom are 100% identical. Not only is another case known, but this pair was the first to be detected before birth.
"The ultrasound of the mother at six weeks showed a single placenta and the placement of amniotic sacs that indicated she expected identical twins," says Nicholas Fisk, a specialist in fetal medicine, who cared for the young family four years ago at Royal Brisbane and Women's Hospital.
"However, a 14 week ultrasound showed that the twins were male and female, which is not possible for identical twins."
Typically, twins are available in only two varieties. There is the non-identical "dizygotic" type, which is the result of two eggs each fertilized by a separate spermatozoon.
Then there are those that are identical, or "monozygotic," in which a solitary fertilized egg divides completely into distinct individuals before settling into its expected program of fetal growth and development.
Before 2007, the very idea of a third "sesquizigotica" category was more theoretical than an established fact. Then came a casual discovery of twins born in the United States that turned out to be genetic chimeras.
Both of these children possessed a mix of cells, some with two X chromosomes and others with a Y chromosome. If one of the children had not been born intersex, it is possible that we would never be wiser to their genetic secret.
Similarly, while none of the Australian twins is physiologically present as intersexual, they both have an assortment of cells carrying pairs of XX or XY chromosomes.
Also the test of the cells taken from their respective pockets of amniotic fluid showed that while the maternal DNA of each was 100 percent identical, only 78 percent of the paternal DNA corresponded.
One possible explanation for this assortment of genomes in a single person is that the mother's eggs may have been copied prematurely before being fertilized by two sperm but did not separate completely.
There is another possibility, one favored by specialists who investigate the most recent case.
"It is likely that the mother's egg was fertilized simultaneously by two father's sperm before dividing," Fisk says.
As that friend who is tagging on a first date, a further selection of genes should mean disaster for any budding love story, which means that a newly fertilized embryo usually does not wait for him to do it.
"In the case of the [Australian] sesquizygotic twins, the fertilized egg seems to have evenly divided the three groups of chromosomes into groups of cells that then split into two, creating the twins, "says clinical geneticist Michael Gabbett of Queensland University of Technology, Australia.
With so few examples to follow, it is difficult to know for sure which explanation is more accurate, or if each pair of twins develops in slightly different ways.
It is also difficult to estimate how many non-identical twins actually share the same selection of mother's DNA.
A survey of global databases on twins suggests if there are others out there, they are nonetheless incredibly rare examples.
"At the outset we questioned whether there were other cases that were wrongly classified or not reported, so we looked at the genetic data of 968 twin brothers and their parents," Fisk says.
"However we found no other sesquizigotic twin in these data, nor any case of semi-identical twins in large global twin studies."
This rarity excludes any case for routine genetic screening for chimerism in twins, at least for now.
Advances in genetic screening and the expansion of databases of medical data could, however, lead to the discovery of more semi-identical twins in the future, and possibly help us to better understand the fertilization process in more detail.
In the meantime, these groups of twins can legitimately claim to be just three of a kind.
This research was published in The New England Journal of Medicine.