Doctors identify first case of semi-identical twins

A strange pregnancy puzzled doctors in Australia. An initial ultrasound showed two embryos sharing a placenta, which normally indicates monozygotic, or identical, twins. But weeks after, another ultrasound revealed the two fetuses had different sexes, a seeming contradiction to the earlier discovery. 

A new study published this February in the New England Journal of Medicine describes the details of the case nearly four years later. Researchers conducted DNA testing on the twins to determine their genetic likeness. The results showed that the twins shared 100 percent of their maternal DNA but only 78 percent of their paternal DNA. Dizygotic, or fraternal twins share usually around 50 percent of both paternal and maternal DNA with each other, no more than non-twin siblings. On the other hand, identical twins share 100 percent of all their DNA. These twins were somewhere in between, known as semi-identical or sesquizygotic twins.

To find out the rarity of this case, researchers also analyzed the DNA of 968 twins along with large sibling databases and found no other cases of sesquizgosity. Twins occur in about three percent of pregnancies in the U.S, and of this three percent, about two-thirds of twin pregnancies are dizygotic. Semi-identical twins have only been reported once before, which occurred in the U.S. in 2007.

So how did this rare case happen? Fraternal twins are caused by two separate ova being fertilized by two sperm. Identical twins occur when one ova is fertilized by one sperm, and the resulting zygote splits into two separate embryos. But neither of these ways can explain the unique genetic overlap in sesquizygotic twins. An alternative explanation was proposed in a different study using a bovine model. The theory is that it was initially caused by two sperms fertilizing one egg. Normally, cellular mechanisms prevent this from happening, and if it does, the triploid fetus (meaning it has three sets of chromosomes) wouldn’t be viable. But, in this case, the genomes are believed to have duplicated and separated into three diploid cells, each with the correct two sets of chromosomes. One of these cells obtained one copy of DNA from each sperm, whereas the other two cells obtained one copy of DNA from the ovum and another from one sperm. These latter two cells were able to survive and develop into two separate twins, sharing the same maternal DNA from the single ovum and having different paternal DNA from the two different sperm.

Semi-zygotic twinning isn’t the only unusual event that can happen in pregnancy. In some instances, identical twins can be “mirror image twins,” caused by late splitting of the embryo. These twins can have opposite traits to each other, such as one being left-handed and the other right-handed. Of course, there is also the case of conjoined twins, which is believed to be caused by partial separation of the fertilized egg.

The sesquizygotic twins in this study, now four years old, are healthy and are meeting their developmental milestones. Their unique condition helped to shed light into how the phenomenon of semi-identical twins can arise.