Monozygotic twins always have the same phenotype. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally from an XXY Klinefelter syndrome zygote splitting unevenly.
看了眼维基:
有纪录的同卵异性的两种情形:
同卵男胎中一方 Y 染色体丢失形成的 45,XO/46,XY,女性一方会长不高
能达成万华镜和缘之空的设定需求的应该是另一种更少见的情形:47,XXY/46,XX,因为很稀有谷歌的前几页只看到一份 case report