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Several years ago, an expecting mother was carrying identical twins: a rare and joyous occasion, but known to happen from time to time. At her 14 week scan, something more remarkable was discovered- one of the identical twins was a boy, and the other a girl.

The surprised mother was told that her babies were ‘sesquizygotic’ or in plainer English ‘semi-identical’. Incredibly, at the time of fertilization, 2 sperm fertilized the same egg which produced three sets of chromosomes (1 from the mother and 2 from the father).

This formed three cells and two of these with the correctly matching DNA started developing into foetuses. Remarkable, and unlike fraternal twins who share 50% of their DNA, the twins are understood to have 100% of their mother’s DNA and 78% identical on their father’s side. Putting them at an obscure percentage of similarity to both fraternal twins and siblings or identical twins.

The only other case recorded of sesquizygotic twins documented that both were intersex. This also applies to the Australian twins. The twins have a certain percentage of XX chromosomes (female) and XY chromosomes (male). The girl has now had her ovaries removed while the boy’s testes are being kept an eye on.

This is a really fascinating area of biology – we encourage all biologists and medics to look into this very rare case to give an incredible interview.

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