Dr Sermed Mezher MBChB (hons), MRes, MRCGP (2023) on Instagram: "Doctor Explains World First Genetics How is this case different from mosaicism? In classic mosaicism, an individual’s cells originate from a single fertilized egg but diverge during early development, so some cells carry one genotype (say XX) and others another (say XY or some other variation). In contrast, chimerism involves two distinct zygotes (or in this case twin fetal cell exchange) resulting in two genetically different cell