Sets of twins are commonly grouped into two classes: monozygotic, or identical; and dizygotic, or fraternal. Identical twins result from a fertilized egg (an egg that has fused with a sperm cell) dividing in two. Both newly identical fertilized eggs can then go onto to develop into their own organisms, sharing identical genes, the functional units of DNA. Fraternal twins are instead the result of two sperm cells each independently fertilizing a separate egg. In this case, the twins share about 50% of their genes, the same as any other set of full siblings, and can have different biological sex, unlike the former case. For the second time ever, a group of researchers in Australia have discovered a pair of “sesquizygotic,” or semi-identical, twins, resulting from a distinct fertilization process.

An ultrasound of a pregnant woman at 6 weeks showed a set of monochorionic twins, which share the same placenta, an observation typically indicative of identical twins. Eight weeks later, however, a subsequent test showed that the twins were of different biological sex. As sex is determined by one’s genes, and identical twins share all their genes, this contradicts the above results, instead suggesting the twins were fraternal. A set of DNA sequencing tests were then conducted, revealing that the twins shared identical sets of maternal DNA but only 78% of paternal DNA, placing them somewhere between identical and fraternal.

A likely cause of the above scenario is polyspermy, in which one egg is fertilized by multiple sperm cells. In this case, the egg carrying the maternal DNA is shared between the twins, while the different sperm cells allow for segregation of paternal DNA such that the set given to each developing fetus is distinct. While biologists have known that eggs can occasionally be fertilized by multiple sperm cells, this often results in an inviable pregnancy. Therefore, an interesting outstanding question is what specific genetic or environmental conditions allowed this unique case, and perhaps the previously documented one, to remain viable?

Managing Correspondent: Andrew T. Sullivan

Press Articles: Semi-identical twins are rare, and doctors say they’ve identified the second case ever, CNN

Doctors say they have identified rare set of semi-identical twins in Australia, USA Today

Extremely Rare Sesquizygotic Twins Identified in Australia, Sci News

Original Journal Article: “Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning,” New England Journal of Medicine

Image Credit: Maria Mellor, Flickr, Under Creative Commons Attribution 2.0 Generic License

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