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Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning. Sesquizygotic multiple pregnancy is an exceptional intermediate between monozygotic and dizygotic twinning. We report a monochorionic twin pregnancy with fetal sex discordance. Genotyping of amniotic fluid from each sac showed that the twins were maternally identical but chimerically shared 78% of their paternal genome, which makes them genetically in between monozygotic and dizygotic; they are sesquizygotic. We observed no evidence of sesquizygosis in 968 dizygotic twin pairs whom we screened by means of pangenome single-nucleotide polymorphism genotyping. Data from published repositories also show that sesquizygosis is a rare event. Detailed genotyping implicates chimerism arising at the juncture of zygotic division, termed heterogonesis, as the likely initial step in the causation of sesquizygosis.