Twins can be fraternal, identical — and in extremely rare cases — semi-identical.
A pair of twins born in January 2014 in Australia share all of their mother's genes, but only 78 percent of their father's, according to a new case report published yesterday (Feb. 27) in The New England Journal of Medicine.
It's unclear how many other semi-identical, or "sesquizygotic," twins are out there, but it's likely "extraordinarily rare," said lead author Dr. Michael Gabbett, the diagnostic genomics course coordinator at the Institute of Health and Biomedical Innovation in Brisbane, Australia. The first set of semi-identical twins was identified in 2007 in the U.S., when they were infants; this is the first time semi-identical twins have been identified in the womb, according to the report. [Seeing Double: 8 Fascinating Facts About Twins]
Initially, the mother of the twins described in the case report thought that she was pregnant with identical twins, based on an ultrasound early in her pregnancy. Later in the pregnancy, however, the woman's doctors were surprised to see that the twins were a boy and a girl. Because identical twins share all of their genes, they can't be of opposite sexes like fraternal twins can.
To analyze the fetuses' genes, the doctors took samples of the amniotic fluid that surrounded each twin. (The twins were in separate amniotic sacs in the womb.) This was how they found out that the twins shared 100 percent of their mother's genes but only 78 percent of their father's.
Normally, a human's DNA comes from two sources: one set of chromosomes comes from the mother's egg and one set comes from the father's sperm. In fraternal twins, two sperm fertilize two separate eggs, yielding twins that share half of their mother’s genes and half of their fathers; in identical twins one sperm fertilizes a single egg, which splits up into the twins that share all of their mother and father’s genes. But in the semi-identical twins, one set of chromosomes came from the egg, and the second set was made up of chromosomes from two separate sperm, Gabbett told Live Science.
But how does this happen? Gabbett hypothesizes that the mother's egg was fertilized by two separate sperm, each carrying its own set of chromosomes –– one set of chromosomes from the mother ended up combining with two different sets of chromosomes from the father.
Once an egg is fertilized, it becomes impenetrable to other sperm, so Gabbett believes that the two sperm must have arrived at the egg at the same time. Any other explanation, such as two sperm fertilizing two eggs with the exact same genetic makeup, "is biologically implausible," he added.
Next, the three sets of chromosomes would've been divvied up into three separate cells: One cell received chromosomes from the mother and first sperm cell, the other from the mother and second sperm cell and the third from both sperm cells. Because a cell needs chromosomes from both the mother and father to survive, that last one would have eventually died. The surviving cells would go on to combine together and then divide again into the two twins.
In addition, the girl and boy each have male and female sex chromosomes — in other words, each twin has some cells that carry an XX pair (female) and some that carry an XY pair (male). Having both male and female pairs of sex chromosomes throughout various cells in the body is linked to certain developmental problems in the reproductive organs as well as cancer, Gabbett said. When the doctors examined the girl twin's ovaries, they found some changes linked to cancer, and so "the difficult decision was made to remove them so she didn't develop cancer," he said.
The girl also developed a blood clot shortly after she was born — blood clots are a common complication for identical twins in general — and the clot cut off the blood supply to her arm. As a result, doctors also had to amputate her arm.
There is good news, however: The twins are now 4 and a half years old and are otherwise developing normally, Gabbett said.
To get a better understanding of how rare sesquizygosis twins are, the researchers went on to examine the genes of nearly 1,000 other twins and didn't find another case of semi-identical twins, nor did they find evidence of the condition in a previous study from another research team that examined DNA from more than 20,000 twins.
Still, it's possible that there are cases doctors have missed: Unless the twins are different genders, sesquizygotic twins could be missed, because people might just assume they're identical twins, Gabbett said. Even so, we "believe this is so rare that routine testing is not warranted," Gabbett said.
Only one other case in the world has been reported, to their knowledge, Gabbett said. Because of its rarity, "some doctors still don't believe that it exists."
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Originally published on Live Science.