Apparently, there is a very rare phenomenon called sesquizygotic or "semi-identical" twins, and two were just born in Brisbane, Australia (the only other known semi-identical twins were born in the United States in 2007).
Twins are normally either identical (where one egg is fertilized by one sperm, but the resulting zygote then splits into two, resulting in two babies with identical genetic material) or fraternal (where two eggs are fertilized by two different sperm, resulting in two babies born in the same pregnancy, but with a genetic make-up no more similar than siblings born to the same parents but at different times).
With semi-identical twins, a very rare sequence of events leads to two sperm fertilizing a single egg. From this, three "genetic packages" arise, but the one that contains two sets of genetic material from the father dies off, leaving the other two to develop fully. What results is a pair of twins that are more genetically similar than fraternal twins, but not completely identical, hence the label "semi-identical". In this case, one twin is a girl and one is a boy.
Twelve years ago, the Anerican semi-identical twins had a bit of a rough go of it, with one twin losing an arm soon after birth due to a blood clot, and then having her ovaries removed at age three to avoid a strong risk of to cancer, both due to the unusual properties of her genetic make-up. The Australian twins can probably look forward to a lifetime of scientific tests and instrusive press coverage, but we should probably give them, and their parents, a break, and not go down that route.
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