The genetics of twinning: from splitting eggs to breaking paradigms.

Over the last decades, there has been a twin boom in terms of live births. Meanwhile, new insights into the genetics of twinning have changed major paradigms. The first major paradigm is that monozygotic (MZ) twins are identical. The second is that twins are either MZ or dizygotic (DZ). The third is that MZ twins happen by chance alone. We have discovered striking epigenetic and other differences between seemingly alike individuals; the presence of fascinating intermediate twin forms; and the potential for familial MZ twinning and even a twinning gene. As this article discusses, the aforementioned paradigm shifts have influenced novel research directions and improved clinical approaches to twin-management.