Pregnancy Ch-037.qxd

INTRODUCTION This chapter introduces the field of genetic research, in which twin studies are a powerful approach to exploring the genetic basis of complex traits. Twin studies take advantage of the existence of mono-and dizygotic (MZ and DZ) twins and use the comparison of resemblances of MZ and DZ twins as the basis for analysis of variations in human traits and diseases. We describe the basic methodology for twin studies, including recent extensions exploiting advances in molecular genetics. We illustrate the value of twin studies by giving a few selected examples from various fields of medicine and psychology, and provide an overview of twin registries which collect data on twins and their relatives worldwide. These registries have been established to obtain an insight into the genetic epidemiology of complex traits and diseases, to study the interaction of genetic factors with sex, age and lifestyle factors and to study the causes of comorbidity between different traits and diseases. Obesity, diabetes, hypertension and psychiatric disorders are examples of common diseases that are a result of various genetic susceptibility factors interacting with environmental risks. Twin registries have been instrumental in establishing the genetic component in susceptibility to these conditions. So far, however, it has been difficult to identify the responsible genes. Because of the design and the often (very) large sample sizes of twin registers, they offer unique opportunities for selected sampling for gene hunting, employing both linkage and association studies. Genetic epidemiology aims to disentangle and quantify the contributions of genes, shared environment , individual-specific environment and their interactions to variations in human traits. Research questions address the etiology of individual differences (what statisticians call variation) in health and disease, or in continuously varying traits such as height, weight or blood pressure. Variation for a trait in a particular population may be caused by genetic differences between individuals and/or by differences in their environment. The effects of genes and environment may be additive, or they may interact with each other. To explore the etiology of individual differences, or the etiology of clustering of these differences within individuals, it is necessary to collect data from non-random samples. The study sample can include relatives who are genetically related but who grew up in unrelated environments (the so-called adoption design), or relatives who grew up in similar environments but who are of different genetic relatedness (for example the twin design). If the exposure to environmental …