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(Newser)
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There are identical twins, there are non-identical twins, and in incredibly rare cases, twins are somewhere in between. The second never recorded case of semi-identical or sesquizygotic twins was described in a recently published 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, head of maternal and twin care, explains how this happened, according to the BBC. "The mother's ultrasound at six weeks showed a single placenta and amniotic sac positioning indicating that she was waiting for identical twins." However, an ultrasound at 14 weeks showed that the twins were men and women, which is not possible for identical twins. "
With identical twins, a fertilized egg splits in two; fraternal twins are the result of the fertilization of two eggs by two spermatozoa. Sesquizygotic twins occur when an egg is fertilized by two sperm before separating. This means that there are three sets of chromosomes – two from the father and one from the mother – and experts say it's very unusual that such embryos survive, Science Daily reports. The twins share more genetically than fraternal twins, but less than identical twins: "The genotyping of the amniotic fluid in each bag showed that the twins were maternally identical but shared 78% of their paternal genome chemically", indicates the study. The other known case was identified in the United States in 2007. (Read more stories about twins.)
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