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

4 thoughts on “A New Kind of Twin

  1. My sister and I were born in Brooklyn in 1949. The doctor told my mother that there was one placenta and therefore we were identical. I have been curious about this since we do not look identical. A few years ago we had our DNA tested and the result came back that we are dizygotic. I have always believed we were something in between, this may be our answer. I would love to find out more about half-identical twins.

    1. This is the same with my twin boys born in 2010. One placenta but look different. One blonde, one brown hair. Of course they look similar. I wonder if 23 and me would be able to answer with whether they are sesquizygotic or not?

  2. I believe my sister and I are semi-identical twins, but I thought this type was identified in the 1970’s! We were born in 1959, and there was one placenta and two sacs. The doctor told my parents that they’d have to wait to see if we were identical or not. We aren’t, just nearly so. We tell everyone we’re Half identical and that our moher’s egg split before fertilization instead of after. Fertilization isn’t what makes the egg split, so it’s just a matter of timing. Makes total sense to us!

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