“Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning”, Michael T. Gabbett, Johanna Laporte, Renuka Sekar, Adayapalam Nandini, Pauline McGrath, Yadav Sapkota, Peiyong Jiang, Haiqiang Zhang, Trent Burgess, Grant W. Montgomery, Rossa Chiu, Nicholas M. Fisk2019-02-28 (heritability)⁠:

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. Detailed genotyping implicates chimerism arising at the juncture of zygotic division, termed heterogonesis, as the likely initial step in the causation of sesquizygosis.

[However] 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.