Not identical, not fraternal, but identical: Australian doctors have encountered in the pregnancy of a 28-Year-old on a small Sensation. In her belly, a girl and a Boy, who had, as identical twins have the identical genetic makeup of the mother – but how fraternal twins of different sperm of the father emerged and grew.
The case of the second in the world, in the researchers, such a semi-identical twins discovered. And it was the first Time that the fact was already detected in the mother’s womb, reports the physicians in the renowned “New England Journal of Medicine”.
The pregnancy was especially, the Doctors noticed very early on. “The ultrasound in the sixth week showed that the mother was expecting two children, but only one placenta had a fruit bubble”, says Nicholas Fisk from the University of New South Wales in Sydney. From this, the physician concluded that in the belly of the 28-Year-old identical twins grow up – fraternal twins develop two placentas.
In the 14. Week but the doctors expected a re-ultrasound to be a Surprise. One of the twins was male, the other female. A circumstance which, in the case of identical twins, actually, impossible. A subsequent genetic study showed The babies were gone, like identical twins from the same egg. This, however, was fused in fertilization with two sperm instead of just one.
For this reason, the maternal genes of the twins were similar, but the paternal but were only 78% identical. This small difference was enough, however, to different sexes.
Identical, fraternal, half-identical – the Overview:
- In the case of monozygotic twins is fertilized a sperm and an egg. The resulting Embryo divides at a very early stage again, so that the two babies grow up. These have a completely identical genetic make-up.
- In the case of dizygotic twins tire instead of an egg two, the independently by two different sperm fertilized. Fraternal twins share, therefore, as all the brothers and sisters approximately half of their genetic material.
- In the case of semi-identical twins, fertilizing two sperm, the same egg cell. The children, with about three-quarters of their DNA: All of the genes that you have inherited from your mother are the same. Those of her father differ due to the two different sperm by up to 50 percent.
Why are semi-identical twins so rare?
In the rule, not the ova continue to develop, if they are fertilized by two sperm. The reason being a genetic surplus: The DNA is distributed to each human on the 23 chromosomes it has two copies. A set of chromosomes he receives at the Merge of the Egg and sperm cell from his mother, from his father.
A set of chromosomes of a healthy human being: Each chromosome occurs twice – once from mother inherited from father.
An egg but two sperm fertilized, the result is that all of the chromosomes present three times: twice by the father and the mother. “This is the life not really compatible,” says Michael Gabbett, a geneticist at the Queensland University of Technology. Usually, the affected embryos develop, therefore, not more.
The Down syndrome – trisomy 21 – is just one of the 23 chromosome three times rather than twice. Already, this leads to enormous development problems of the child.
The semi-identical twins from Queensland, lived still further, explain the researchers, with the amazing sorting operations in the fertilized egg.
- In a first step, the egg doubled the available chromosomes, as do all the cells before they divide. Thus, each chromosome was six times run in the cell: two from the egg, twice from each sperm.
- Then, the fertilized egg does not split as usually into two new cells, but formed three new cells. To this, she divided the six times the finishes of the chromosomes evenly, so that at the end in each cell, only two versions of each chromosome templates – the way it be.
In this way, the egg is a precondition for healthy life. To grew up to this time no twins in the belly of the mother. This had to happen, what is the single most leagues twins at the beginning: After the fertilized egg cell, a small cell cluster is grown, divides this in two embryos, which develop independently of each other to two children.
A Mix of X – and Y-chromosomes
Since in the case of semi-identical twins in Australia at this time, the cells with the genetic information from two sperm long been mixed, carrying both children, the DNA from two sperm. This was a big Problem. One of the sperm contained an X chromosome, resulted in the female cells. The other sperm contained a Y-chromosome, showed so male cells.
Both children wear, therefore, both male and female cells. The now four-year-old Boy was lucky, he is so far grown all healthy. The girl developed hives, a malformation of the eggs, so they had to be removed at the age of three years. Apart from that, both children had mastered all the important steps in the development of normal, the researchers write.
So far, only another case of half-identical twins is known. Doctors in the USA had been trimmed in 2007, as one of two twin children, with no clear gender to the world. In the case of a genetic study showed that the maternal DNA of both twins is identical, the of the father difference, but in Parts.
“We asked ourselves whether there might be other cases that were not correctly identified or reported,” says Fisk. For this reason, the Australian researchers analyzed the genetic material of 968 supposedly fraternal twins. “In these data, we found no further cases,” says Fisk.
Summary: live In Australia twins from an egg emerged, which of the two sperm at the same time fertilized. Usually, embryos will die when this happens. In this case, the development of delivery but not otherwise. As a result, the children carry both female and male cells in your body.