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Between Fraternal and Identical

A pair of semi-identical twins — the second known case — was born in Australia a few years ago, with doctors just now revealing the details, New Scientist reports.

In a case report appearing in the New England Journal of Medicine, the University of New South Wales's Nicholas Fish and his colleagues describe how this set of sesquizygotic twins was discovered: at about six weeks' gestation, the mother was thought to be pregnant with monozygotic twins, as they shared a placenta, but after 14 weeks, the twins were noted to be sex discordant.

Following this discovery, the clinicians sampled the twins' amniotic sacs as well as samples from their parents to work out what was going on. After genomic analysis using a combination of SNP karyotyping, fluorescence in situ hybridization, and sequencing, Fish and his colleagues uncovered XX/XY chimerism in the twins. They shared identical maternal genotypes, but had different paternal ones, leading the team to suggest that a single egg was fertilized by two sperm and that then developed into twins.

"We traditionally categorize twins as either identical or non-identical, and this is a third type of twinning characterization," first author Michael Gabbett from Queensland University of Technology tells the Guardian.