In 2014, doctors caring for a pregnant women in Australia noticed that the twins she was carrying shared a single placenta, which indicates identical twins- twins formed when a fertilized egg splits in two. But tests also showed they were a boy and a girl, which means fraternal twins- formed from two different fertilized eggs. They are a third type of twin, called sesquizygotic, meaning half-identical. They are identical on their mother's side and fraternal on their father's side.
...in this case, the authors theorize, two sperm cells simultaneously fertilized the same egg. Ordinarily, that sort of mistake quickly results in a miscarriage, since humans usually can’t develop with three different sets of chromosomes. Somehow, though, the resulting zygote incorporated an equal split of DNA from all three sets, with three groups of cells forming afterward: Cells containing the mother’s DNA and DNA from sperm 1; cells with the mother’s DNA and DNA from sperm 2; and cells containing DNA from only sperm 1 and 2. Over time, the third group of sperm-only cells was effectively crowded out by the cells containing DNA from both parents. Then, even more unexpectedly, the bundle of cells divided into two embryos, creating the twins.
“They are 100 percent identical on the mother’s side and 78 percent identical the father’s side, so this averages out to being 89 percent identical,” lead author Michael Gabbett, a geneticist at Queensland University of Technology, told Gizmodo.
Half-identical twins are so rare that researchers could find only one other documented case. The twins, now four years old, are chimeras, both containing different combinations of their father's DNA in their various cells. Read more about the sesquizygotic twins at Gizmodo.
(Image credit: Flickr user Maria Mellor)