Most monozygotic twins are remarkably similar.
It is usually assumed that monozygotic twins always share the same genome and that their hereditary endowments are the same. There are several cases in which identical twins can have discordant phenotypes. The major routes involve aneuploidy, X-chromosome inactivation differences, imprinting, and circulatory differences. For example, take the situation of an egg fertilized by a Y-bearing sperm. In that case, the twins would have different chromosome complements.