First semi-identical twins

The first documented case of semi-identical (sesquizygotic) twins was reported in the USA in late 2006. They share 100% of the genes from their mother but only part of their father's DNA, meaning they are more like siblings on his side. The unprecedented case was reported in the journal Human Genetics on 13 December 2006.

Until that point, it was thought there were only two types of twinning: dizygous (fraternal) - twins that are conceived from separate eggs and sperm and develop separately in the uterus - and monozygous (identical) - those that result from the splitting of a single fertilized egg. Sesquizygotic is now a recognized third form.

A second case of sesquizygotic twinning was identified via genetic testing during pregnancy in a pair of twins from Queensland, Australia, in 2014. The case was detailed in The New England Journal of Medicine in February 2019.