ABSTRACT
For years it was commonly believed that approximately 25% of monozygotic (MZ) twins exhibited what was loosely referred to as the ‘mirror phenomenon’. In this situation, what was seen anatomically on the left side of the body in one twin was seen on the right side of the other twin. Mirror-image features in twins commonly include moles, birthmarks, tooth eruption patterns and hair whorls. According to some authors, the mirror phenomenon is also a partial explanation for why more than a third of MZ twins are lefthanded, double the rate in the general population. In contrast, only rarely are the internal organs reversed in one of the twins (situs inversus)1.
