In 2019, an extremely rare case of semi-identical twins was identified in the womb. The mother of the twins was told she was pregnant with identical twins after an ultrasound. However, the twins were later determined to be male and female. Because identical twins share all of their genes, they cannot be opposite sexes. To understand what happened, doctors took samples of the amniotic fluid. They found that the twins shared 100% of their mother’s genes, but only 78% of their father’s. This means that two separate sperm fertilized one egg at the exact same time.
To get a better understanding of how rare sesquizygotic twins are, the researchers went on to examine the genes of nearly 1,000 other twins and didn’t find another case of semi-identical twins, nor did they find evidence of the condition in a previous study from another research team that examined DNA from more than 20,000 twins.
Only one other case of sesquizygotic twins has been reported in the medical literature.