ABSTRACT
Twins are classified into two major groups, dizygotic (DZ), which result from two different ova fertilized by two different sperm, and monozygotic (MZ), which derive from a single ovum fertilized by one sperm that divides to form two embryos1. The accepted origin of MZ twins from a single fertilized ovum implies that their genetic constitution should be identical. This fact underlies the lay terminology of ‘identical twins’. On the other hand, DZ twins, originating from two independently fertilized ova, should be no more alike than any pair of siblings, and are characterized by the lay public as ‘fraternal twins’2. As early as 1875, Galton postulated that any difference observed within an MZ pair should depend on environmental influences, whereas differences within a DZ pair should be attributable to both environmental as well as genetic factors3. This concept soon led to the widespread use of twins to compare the contribution of nature (heredity) versus nurture (environment) in the study of congenital anomalies, complex traits and diseases4. Indeed, this comparison has been useful in identifying the heritability of a specific characteristic. Although it is true that the vast majority of MZ twins are genotypically as well as phenotypically identical, several recent studies show that significant numbers of MZ pairs differ in their genotype as well as in their phenotype, or only in their phenotype (for more details see Hall5,6). This chapter focuses on the mechanisms causing genotypic and phenotypic discordance in monozygotic twin pairs.
