Moisaicism in the human preimplantation embryo

Recent studies have revealed widespread mosaicism in the human preimplantation embryo at the nuclear and chromosomal level arising at fertilization and preimplantation development. Molecular cytogenetic analysis by fluorescent in situ hybridization (FISH), in particular, has for the first time made it possible to analyse all or almost all nuclei in cleavage stage embryos. The genotype of an individual is therefore not strictly defined at conception by the genotype of the germ cells. Rather it seems (at least in vitro) that the processes of fertilization, syngamy and the postzygotic mitotic divisions can be irregular. In some cases, therefore, the eventual genotype only emerges by a process of natural selection.