There are identical twins, there are non-identical twins—and, in unbelievably rare cases, there are twins that fall somewhere between the two. The second case ever recorded of semi-identical, or sesquizygotic, twins was described in a recently released study published in the New England Journal of Medicine. The twins, born in Brisbane, Australia, in 2014, were the first of their kind to be identified during pregnancy. Professor Nicholas Fisk, who was in charge of the care of the mother and twins, explains how that happened, the BBC reports. "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."
With identical twins, a fertilized egg splits in two; fraternal twins are the result of two eggs being fertilized by two sperm. Sesquizygotic twins occur when an egg is fertilized by two sperm before splitting. That means there are three sets of chromosomes—two from the father and one from the mother—and experts say it is very unusual for such embryos to survive, Science Daily reports. The twins have more in common with each other genetically than fraternal twins, but less than identical twins: "Genotyping of amniotic fluid from each sac showed that the twins were maternally identical but chimerically shared 78% of their paternal genome," reads the study. The other known case was identified in the US in 2007. (Read more twins stories.)