These Twins Have Strange Semi-Identical DNA, in Only The Second Case Ever Discovered

These Twins Have Strange Semi-Identical DNA, in Only The Second Case Ever Discovered
An illustration of human sperm competing to fertilize an egg. Illustration: Maria Mellor ( CC BY 2.0 )

A pair of Australian brothers have become part of some of the rarest human beings documented, according to doctors. They say that the brother and sister are semi-identical twins, who share exactly the same DNA on their mother’s side, but only part of their father’s genetic makeup. It is believed that children are the second case of semi-identical twins discovered, and the first to be identified during pregnancy.

The story of the twins was detailed in a case study published Wednesday in The New England Journal of Medicine. According to the authors, siblings are a class of twins called sesquizigóticos, something like a midpoint between fraternal and identical twins.

Fraternal twins occur when two different sperm fertilize two different eggs, creating two different zygotes, which end up implanted in the uterus as developing embryos. Therefore, they are as genetically similar (approximately 50%) as would be two average siblings: newborns at the same time. Identical twins, on the other hand, occur when the same sperm fertilizes the same egg, but then that zygote is divided into two embryos, each of which shares the same genetic mix of DNA from mom and dad.

According to the authors, the brothers are a class of twins called sesquizigóticos, something like a midpoint between fraternal and identical twins

But in this case, the authors theorize that two sperm cells fertilized the same ovule simultaneously. Typically, that kind of error quickly translates into a miscarriage, since humans generally can not develop with three different sets of chromosomes. However, somehow, the resulting zygote incorporated an equitable division of the DNA of the three sets, with three groups of cells forming afterwards: cells that contain the DNA of the mother and the DNA of the sperm 1; cells with the DNA of the mother and the DNA of the sperm 2; and the cells that contain DNA from only sperm 1 and 2. Over time, the third group of cells that only contain sperm was displaced by cells that contain DNA from both parents. Then, even more unexpectedly,

” They are 100% identical on the maternal side and 78% identical on the father’s side, so this average is 89% identical, ” senior author Michael Gabbett, a geneticist at the Queensland University of Technology, told Gizmodo.

Gabbett and his co-author, Nicholas Fisk, an obstetrician and vice chancellor of research at the University of New South Wales, helped manage the fetal care of the twins at the Royal Brisbane and Women’s Hospital in Queensland in 2014. Although it was evident within weeks of the pregnancy in which the mother had twins, it was not until the beginning of the second trimester that the team noticed something unusual.

” The ultrasound of the mother at six weeks showed a single placenta and the placement of amniotic sacs that indicated that she was expecting identical twins,” Fisk said in a statement. ” However, an ultrasound at 14 weeks showed that the twins were male and female, which is not possible for identical twins .”

Fisk and his team also searched for other cases of semi-identical twins in the medical literature and in the genetics databases of twins. But they only managed to find another instance. That case, published in 2007 by doctors in the United States, highlights the complexity of biological sex.

US doctors reported that the division of paternal chromosomes resulted in a twin being intersex, meaning they had ambiguous genitalia; the other was anatomically masculine. The Y chromosome is usually what determines the sex of a person, and biological males have predominantly XY chromosomes in their cells.

It turned out that both twins had a mix of sex chromosomes XY and XX, but the intersexual twin had only a 5% XY split compared to XX, while the anatomically male twin had a 50-50 split. The anatomically masculine part of the XX genes was not enough to prevent them from developing male sex organs.

” At first we asked if there were other cases that had been misclassified or not reported, so we examined genetic data on 968 twins and their parents, ” said Fisk. ” However, we did not find other sesquizigotic twins in these data, nor any cases of semi-identical twins in large studies of global twins .”

The Australian twins are considered chimeras by their father’s side, a twin has a division of approximately 50-50 of the sex chromosomes XY and XX, and the other has a division of approximately 90-10 from XX to XY. However, unlike the case of 2007, the twins are anatomically male and female, respectively.

Unfortunately, the twin had an unrelated blood clot shortly after birth, one that required an amputation of her right arm beyond her shoulder. Later, he was also diagnosed with gonadal dysgenesis, a congenital condition related to cases of rare chromosome configurations. The disorder causes a person’s genitals not to develop normally and have a high probability of becoming cancerous,

All in all, both twins, now 4 years old, seem to be developing normally and functioning very well, Gabbett told Gizmodo. He and his team plan to continue monitoring the twins in the face of any other complication.