Monozygotic twins discordant for trisomy 21: Discussion of etiological events involved

Our case of monozygotic twin with discordant trisomy 21 might start with a prezygotic maternal meiosis II non-disjunction error-caused trisomy 21 zygote, and after twinning, one remained trisomy 21, and the other twin underwent trisomy rescue and became a mosaic trisomy 21 in morula or early blastocyst stage before the formation of pre-embryo, which subsequently resulted in mosaic trisomy 21 of the placental tissue and normal chromosomal constitution of the fetus.