The Gizmodo article Australian Siblings Are Semi-Identical Twins, Some of the Rarest Humans Ever links to the new paper in NEJM Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning Gabbett et al. N Engl J Med 2019; 380:842-849 DOI: 10.1056/NEJMoa1701313.
The abstract of this paper says:
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.
There are several terms I am trying to understand here:
- sesquizygosis and sesquizygosity: A postulated form of twinship, in which individuals share between 50 and 100% of genetic identity by state (number of shared alleles).
- heterogonesis: The assortment of two or more paternal genomes into distinct cell lineages after polyspermic fertilization of an ovum.
- juncture of zygotic division (?)
So far I understand that the chimerism means for each fetus, some cells have a distinct genome from other cells, and sesquizygosity means (at least) that there is DNA present from more than one sperm.
But I don't understand what "...chimerically shared 78% of their paternal genome..." means exactly when speaking about twins. What does chimerical sharing mean in this case? Are we talking about four genomes (two genomes in each of two fetuses)? If so, how does that comparison reduce to a single value of 78%?
Heterogonesis \ he-tə-rō-gō-ˈnē-səs \
The assortment of two or more paternal genomes into distinct cell lineages after polyspermic fertilization of an ovum.