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.

2C nuclear DNA amounts for 24 species of Lathyrus (section Lathyrus) were determined using flow cytometry. A greater than two-fold variation was observed, ranging from 10.2 pg in L. basalticus to 24.2 pg in L. latfolius. In general, the perennial species had greater DNA amounts than the annuals. Significant intraspecific variation was observed in five species of Lathyms (from 10.1% in L. annuus to 28% in L. tingitanus). A positive correlation was observed between DNA values obtained by flow cytometry and those previously determined by microdensitometry. Finally, the distribution of DNA amounts in species within section Lathyrus appears to be continuous. 0 1997 The Linnean Society of London

No abstract