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In 1984, Hrubec and Robinette published what was arguably the first review of the role of twins in medical research. The authors acknowledged a growing distinction between two categories of twin studies: those aimed at assessing genetic contributions to disease and those aimed at assessing environmental contributions while controlling for genetic variation. They concluded with a brief section on recently founded twin registries that had begun to provide unprecedented access to twins for medical research. Here we offer an overview of the twin research that, in our estimation, best represents the field has progress since 1984. We start by summarizing what we know about twinning. We then focus on the value of twin study designs to differentiate between genetic and environmental influences on health and on emerging applications of twins in multiple areas of medical research. We finish by describing how twin registries and networks are accelerating twin research worldwide. [ABSTRACT FROM AUTHOR]

Prolonged fatigue equal to or greater than 1 month duration and chronic fatigue equal to or greater than 6 months duration are both commonly seen in clinical practice, yet little is known about the etiology or epidemiology of either symptom. Chronic fatigue syndrome (CFS), while rarer, presents similar challenges in determining cause and epidemiology. Twin studies can be useful in elucidating genetic and environmental influences on fatigue and CFS. The goal of this article was to use biometrical structural equation twin modeling to examine genetic and environmental influences on fatigue, and to investigate whether these influences varied by gender. A total of 1042 monozygotic (MZ) twin pairs and 828 dizygotic (DZ) twin pairs who had completed the University of Washington Twin Registry survey were assessed for three fatigue-related variables: prolonged fatigue, chronic fatigue, and CFS. Structural equation twin modeling was used to determine the relative contributions of additive genetic effects, shared environmental effects, and individual-specific environmental effects to the 3 fatigue conditions. In women, tetrachoric correlations were similar for MZ and DZ pairs for prolonged and chronic fatigue, but not for CFS. In men, however, the correlations for prolonged and chronic fatigue were higher in MZ pairs than in DZ pairs. About half the variance for both prolonged and chronic fatigue in males was due to genetic effects, and half due to individual-specific environmental effects. For females, most variance was due to individual environmental effects. [ABSTRACT FROM AUTHOR]

The University of Washington Twin Registry is a community-based registry of twins identified from the Washington State Department of Licensing. A fortuitous quirk in the Washington State drivers' license and identification number assignment, and collaborative Washington State laws, permitted us to build a statewide registry. Since obtaining approval from the Washington State Attorney General, the Washington State Department of Licensing has provided us with information on over 26,000 newly licensed twins, and we continue to receive computerized records on approximately 80 new twins per week. The University of Washington Twin Registry is assembled by mailing each twin a recruitment packet that includes an information sheet, brochure, brief survey, and nominal gift. Once both members of a twin pair have completed the packet, the pair is enrolled in the registry. As of June 2006, 2287 adult twin pairs have enrolled in the registry; about one half of these are female-female pairs. Among all twins, the average age is 32 years. Based on self-report, 52% of twins are monozygotic, 42% are dizygotic, and zygosity on 6% cannot yet be determined. We also have instituted a clinical protocol for collecting additional data and DNA from all twins participating in research studies requiring an in-person visit. The registry has established policies and procedures to protect the confidentiality of twin data and guidelines for the use of the registry by investigators. Plans for the further growth of the University of Washington Twin Registry and its use are discussed. [ABSTRACT FROM AUTHOR]