Conclusion

Twin studies provide a unique approach to investigating the determinants of a disease or condition. A single twin study cannot absolutely determine the importance of genetic or environmental factors. However, the twin study method, in combination with other approaches, can be a powerful tool for unraveling the causes of disease. see also Behavior; Fertilization; Gene and Environment; Gene Discovery; Inheritance Patterns.

Caroline M. Tanner and Richard Robinson

Bibliography

Bouchard, T. J., et al. "The Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart." Science 250 (1990): 223-228.

Segal, Nancy L. Entwined Lives: Twins and What They Tell Us about Human Behavior. New York: Plume, 2000.

Wright, Lawrence. Twins: And What They Tell Us about Who We Are. New York: John Wiley & Sons, 1997.

Internet Resource

Minnesota Twin Family Study. University of Minnesota. <http://www.psych.umn .edu/psylabs/mtfs/default.htm>.

cell or organism

Viroids and Virusoids

Viruses are infectious agents consisting of a nucleic acid genome made of DNA or RNA, a protein coat, and sometimes lipids. They are able to repli-genome the total cate only inside cells, and the viral genome contains genes coding for pro-

genetic materia| in a teins. Viroids and virusoids are also infectious agents, but they differ from viruses in several ways. For instance, they have a single-stranded circular, RNA genome. Their genomes are very small and do not code for proteins. Viroids replicate autonomously inside a cell, but virusoids cannot. Rather, virusoid replication requires that the cell is also infected with a virus that supplies "helper" functions.

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