An extremely rare case: they document a third type of "almost" identical twins – 14/03/2019

By Heather Murphy

It only took one look at the ultrasound for the doctor to know that he was seeing identical twins.

The position of the two amniotic sacs attached to the placenta was what revealed it. He could have a few months before he can tell the mother when he expects to have two boys or two girls.

What seemed certain is that their babies would share the genre. Nevertheless, at fourteen weeks, when Nicholas Fisk, a specialist in maternal fetal medicine at the time at Royal Brisbane and Women's Hospital, inspected the ultrasound and saw something that puzzled him.

Child. A girl.

That they shared a single placenta.

"It does not make sense," recalls Fisk, who is saying at that time.

It turns out that the twins are neither fraternal nor identical. They enter a third and rare category called semi-identical or sesquicigotic twins. Although it will take several years to prove it, he saw the first semi-identical twins be identified during pregnancy, according to an article recently published in the New England Journal of Medicine.

The couple share one hundred percent of their mother's DNA, but they only have one part of their father's DNA in common.

"They are about three-quarters identical," Fisk said, adding that "it's almost like finding a needle in a haystack".

Typically identical twins are created when a single egg fertilized by a single sperm splits into two. What Fisk suspected of happening in this case was that Only one egg was fertilized by two spermatozoa before dividing.

This should not be possible.

"When a sperm gets into the egg, the membrane closes to prevent any other sperm from entering it," said Fisk, currently badistant vice-rector for research at the University. from New South Wales. "Even if another sperm gets into the egg, you would have three groups of chromosomes and you would not survive."

Even so, when he and his colleagues began exhaustive badysis of each chromosome in the boy and girl, they discovered two healthy babies. Each contained the normal number of chromosomes. However, there were areas in which their DNA was identical and others in which it was different.

This is not something unpublished. Although no doctor has identified sesquicigóticos twins during pregnancy, a doctor in the United States has documented the first case of semi-identical infants in 2007, according to the article.

In 2007, the girl had ambiguous badia, which led her parents to take her to the doctor. Like the Australian twins, the semi-identical American couple was identical on the mother's side but only shared half of the father's DNA. In the Australian case, none of the twins had ambiguous bads.

"At 4 years old, they are doing well and responding to indicators of their development," said Fisk.

Mindy Christianson, Assistant Professor at the Faculty of Medicine of Johns Hopkins UniversityHe said the new case raises important questions about fertilization opportunities.

"The recent case was detected because the twins are a boy and a girl," he wrote in an email. "There could be other same-bad twins out there who are also semi-identical."

Since the publication of the study, Fisk has been inundated with issues of twins under the age of 70 who wonder if they may have spent their entire lives without realizing that they were semi-identical.

To determine if this phenomenon could be more common than expected, Fisk badyzed the genetic data of nearly a thousand fraternal twins. He did not find any semi-identical twin among them.

All this reinforces what Fisk said he loved about the study of twins: there are still so many mysteries to explore. Even the question of exactly how identical twins are formed, he said, has yet to be solved conclusively.

c.2019 New York Times News Service