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Semi-identical Twins: Three Sets of Chromosomes, Two Babies

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An increase in the number of individuals seeking reproductive therapy, coupled with the increasing average age that women are choosing to have babies, has seen an associated rise in the number of multiple births.

In the US, approximately 33.3 per 1000 births in the US are twins1, accounting for over 90% of multiple births (based on data collected from 1980 to 2017).

Types of twinning

Typically, twins are known to be either monozygotic or dizygotic: 

Monozygotic twins form from a single fertilized egg that splits after fertilization. These types of twins are identical, the same gender, and share the same genetic background.

Dizygotic twins form from two separate eggs that are fertilized by separate sperm. Commonly known as fraternal twins, they are the same genetically as siblings, but share a womb.

However, in 2007 this long-standing concept was challenged with the discovery of sesquizygotic twins2. These types of twins are somewhere between fraternal and identical, in that they are semi-identical. Scientists hypothesize that they are formed when an egg fuses with two sperm cells to create a triploid cell, containing three lots of chromosomes.

Sesquizygotic twins in the womb

Now, in a world-first, semi-identical twins have been identified in the womb through genetic testing3. The case is reported in The New England Journal of Medicine by fetal medicine specialist and deputy vice-chancellor (research) at UNSW, Professor Nicholas Fisk, and Queensland University of Technology (QUT) clinical geneticist and diagnostic genomics course coordinator, Dr Michael Gabbett.

"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins. However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins” says Professor Fisk in a recent press release.

The twins, now four years old, are monozygotic on their mother’s side, sharing 100% maternal DNA, however on their father’s side they share only a proportion of the DNA.

“Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion rather than 100 per cent of the same paternal DNA” says Gabbett. 

Sesquizygotic twins are a medical marvel in the fact that three sets of chromosomes are typically incompatible with life. Embryos carrying this number of chromosomes typically do not survive pregnancy.

"In the case of the Brisbane sesquizygotic twins, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins” comments Gebbett.

Sesquizygotic twins – a rarity

Is it possible that sesquizygotic twins are common throughout populations but go undetected? Fisk suggests not. “We at first questioned whether there were perhaps other cases which had been wrongly classified or not reported, so examined genetic data from 968 fraternal twins and their parents."

The researchers found no data pointing to the existence of sesquizyogtic twins, and therefore suggest that the formation of sesquizygotic twins is a rarity.

"We know this is an exceptional case of semi-identical twins. While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing."


1. Statista. (2019). Twin birth rate U.S. 1980-2017 | Statistic. [online] Available at: https://www.statista.com/statistics/276017/us-twin-birth-rate/ [Accessed 27 Feb. 2019].

2.  Souter, V., Parisi, M., Nyholt, D., Kapur, R., Henders, A., Opheim, K., Gunther, D., Mitchell, M., Glass, I. and Montgomery, G. (2006). A case of true hermaphroditism reveals an unusual mechanism of twinning. Human Genetics, 121(2), pp.179-185.

3.Gabett et al. 2019. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning. The New England Journal of Medicine. http://dx.doi.org/10.1056/NEJMoa1701313.