Monozygotic twins: Ten reasons to be different

Abstract

MZ twins, though arising primarily from a single zygote and considered genetically identical, will not ever be absolutely the same. Post-fertilization events such as chromosomal mosaicism, skewed X-inactivation and imprinting mechanisms, as well as other epigenetic mechanisms are responsible for the putative differences between MZ twins. Numerous discordant MZ twins have been reported in the literature, including discordance for malformations or genetic diseases, lateral asymmetry, discrepant growth and intrauterine death of the co-twin. This discrepancy for the disorder may be valuable in the analysis of the effect of a disease upon gene expression or on phenotype variation, and have long-term implications. With today's whole-genome sequencing technologies and increasing evidence for genetic and epigenetic differences in some MZ twins, the understanding of these differences is a whole new field of research.We reviewed the genotypic and phenotypic differences between MZ twins and discuss ten main reasons for being different.