The Chimera twins

Australian doctors have described a unique case of "semi-identical" twins

Sergey Vasiliev, Naked Science

Approximately every 50th pregnancy ends with the birth of twins. They can be identical if they grow from the same egg and the same sperm. The fertilized embryo, divided in two, leads to the appearance of a pair of monozygotic twins, genetically identical. Heterozygous twins are genetically no closer to each other than ordinary brothers and sisters. They are born from different eggs fertilized by different spermatozoa, which then developed during the same pregnancy.

There is also a more complex mechanism that in the rarest cases leads to the birth of "semi-identical" twins, and in 2007 a case of unique "semi-identical semi-identical twins" was described. It was discovered during the study of a child with defects in the development of external sexual characteristics. It turned out that the mother's egg was fertilized by two paternal spermatozoa and thus received a triple set of chromosomes. After division, embryos containing both genetically female and male cells were formed, leading to the birth of twins with a complex form of hermaphroditism.

And in 2014, doctors from the Australian Royal Brisbane and Womans Clinic discovered a second such case, now in the early stages of pregnancy of a 28-year-old woman.

Following the development of twins, scientists described this case in detail in an article published in The New England Journal of Medicine (Gabbett et al., Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning).

Usually such triploid anomalies die quickly, being discarded by the maternal organism. However, in this case, the zygote was preserved and was able to split into two groups of cells, which continued to develop into two embryos. They contain the same set of maternal chromosomes, but different combinations of paternal ones. "At the sixth week of pregnancy, the amniotic membranes visible on the ultrasound of the mother showed that she was expecting identical twins," says Professor Nicholas Fisk. "However, ultrasound at week 14 showed that the twins are a boy and a girl, which is impossible for identical twins."

The formation of semi-identical twins.
1. One egg is fertilized by two sperm cells.
2. The zygote receives one set of chromosomes from the mother and two from the father.
3. Division leads to the appearance of cells with different sets of chromosomes.
4. Cells divide.
5. The embryo is divided into groups of cells.
6. The embryos formed from them develop in parallel.
©The Guardian , The New England Journal of Medicine.

Taking samples of amniotic fluid, the scientists conducted their genetic analysis, finding out the amazing features of these embryos. Each of them turned out to be a genetic chimera and had both cells containing two sex chromosomes X ("female") and containing XY ("male"). Today, children are four years old, their life is by no means simple. One of them received more XY cells and generally developed according to the male type, the second - according to the female type. The girl is experiencing a number of health problems, which is why she had to have her ovaries removed and (after the formation of a blood clot) her arm.

According to the authors of the work, the study of medical databases on twins showed how rare a case they were able to observe. They examined almost 1,000 suspicious testimonies, but none of them managed to conclude that the twins were "semi-identical." So, scientists say, young mothers should not worry about the possibility of giving birth to such twins. The probability of this is too small.

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