Doctors discover a new type of twin: Semi-identical

We’ve heard of identical twins and fraternal twins already. But brace yourselves because a few Australian scientists have identified a new type of twins- semi-identical! Researchers from the UNSW (University of New South Wales) Sydney and QUT (Queensland University of Technology) identified this extremely rare case through genetic testing. Even though there have been some rare reported cases of semi-identical twins, the Queensland twins are the first to be diagnosed while still in the womb. The case of semi-identical twins occurs when the mother’s egg is fertilized by two of the father’s sperms before splitting (1).

At around 6 weeks, the mother’s ultrasound scan hinted that she was going to have identical twins. However, at 14 weeks, the scan results showed that she was having male and female babies, which is impossible in the case of identical twins. Identical twins happen when the mother’s egg splits into two after getting fertilized by one of the father’s sperms. This means that both the identical twin babies are of the same gender. Fraternal twins are conceived when two of the mother’s eggs are fertilized by two of the father’s sperms (2).

Fraternal twins are known as dizygotic; identical twins are called monozygotic, and semi-identical twins are called sesquizygotic. While fraternal twins share half of all their DNA and identical twins share all their DNA, semi-identical twins share three by fourth of their DNA. The Australian twins, a boy and a girl, is the first such case in the country. It was found that after the fertilization of one egg by two sperms, the egg split into three embryos (3). These three embryos had DNA from two sperms, but one of them did not develop after the splitting. The rest of the two developed, and the rest is history!

The first case of sesquizygotic twins was reported in the US in 2007. However, in this case, the doctor identified that the kids were semi-identical twins only after birth. An investigation was conducted on the mixed chromosomes of the babies due to ambiguous genitalia in one of them. This is when doctors realized that they had discovered a new type of twins. Even in the case of the second pair of twins, the ovaries of the girl baby was removed since it was suspected that she was at a higher risk of cancer. Also, the genitals of the boy baby are still being monitored to detect any abnormalities (4).

The second case was reported in 2014 and the twin boy and girl are 4- 1/2 years old now. They have achieved all developmental milestones and are completely healthy. The twins share 100% of their mother’s genes, but they share only a portion of their father’s genes which makes them similar to siblings on the paternal side. A male DNA contains XY chromosomes and a female DNA contains YY chromosomes. When they have a child together, the baby’s DNA is made of one chromosome from the mother and one from the father. This determines the gender of the baby. In this case, one of the father’s sperm contained an X chromosome and the other one contained a Y chromosome which is why even though the twins are identical, they are of different gender (5).

The 28-year-old mother conceived the twins through natural means (6). We can assume that this is an intermediate type of twin case when considering the rest of the two types.

The process of procreation is always a fascination to people who are interested in Science. The case of semi-identical twins is extremely rare and this calls for further research in the area. Hopefully, in the future, we will learn more such interesting facts about semi-identical twins.

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