ABSTRACT
Role of Environment ........................................................................ 4 1.3 Phenotypic Variation in Genetically Identical Animals:
“The Third Component” .................................................................. 6 1.4 The Epigenetic Perspective .............................................................. 9 Keywords ................................................................................................ 16 Acknowledgment .................................................................................... 16 References ............................................................................................... 16 Credits ..................................................................................................... 20
1.1 INTRODUCTION
Identical human twins have been a source of superstition and fascination throughout human history, from Romulus and Remus, the mythical founders of Rome, to movies such as Cronenberg’s “Dead Ringers,” to the twin paradox in the theory of special relativity [1]. For biologists and psychologists, twins have been an important resource for exploring the etiology of disease and for understanding the role of genetic and environmental factors in determining phenotype, and this fundamental question was first enunciated in its alliterated form by Galton in the 19th century [2]. The relative contribution of nature versus nurture can be estimated by comparing the degree of phenotypic similarities in monozygotic (MZ) versus dizygotic (DZ) twins. MZ twins arise from the same zygote, whereas dizygotic twins arise from a pair of separate eggs, fertilized by two different sperm. As a result, MZ twins have the same chromosomal DNA sequence, except for very small errors of DNA replication after the four to eight cell zygote stage. MZ twins share all of their nuclear DNA, whereas DZ twins share only 50% of DNA sequence variation, on average. Therefore, the degree of genetic contribution to a given phenotype can be estimated from the comparison of MZ to DZ concordance rates or intra-class correlation coefficients. Traits that show higher MZ versus DZ similarity are assumed to have a genetic component because the degree of genetic sharing and the degree of phenotypic similarity are correlated. The amount of genetic contribution can be expressed as the ‘heritability’ (h2), which is calculated in various ways (e.g. as twice the difference between the MZ and DZ concordance rates) [3, 4].
