Phenotypic differences in genetically identical organisms: the epigenetic perspective
1The Centre for Addiction and Mental Health, Departments of Psychiatry and Pharmacology, and the Institute of Medical Science at the University of Toronto, Ontario, Canada and 2Department of Psychiatry and Psychology, University of Minnesota, Minneapolis, MN, USA
* To whom corresponding should be addressed at: The Krembil Family Epigenetics Laboratory, Centre for Addiction and Mental Health, 250 College Street, Toronto, ON M5T 1R8, Ontario, Canada. Tel: +1 4165358501; Fax:+1 4169794666; Email: arturas_petronis{at}camh.net
Received January 26, 2005; Accepted February 24, 2005
Human monozygotic twins and other genetically identical organisms are almost always strikingly similar in appearance, yet they are often discordant for important phenotypes including complex diseases. Such variation among organisms with virtually identical chromosomal DNA sequences has largely been attributed to the effects of environment. Environmental factors can have a strong effect on some phenotypes, but evidence from both animal and human experiments suggests that the impact of environment has been overstated and that our views on the causes of phenotypic differences in genetically identical organisms require revision. New theoretical and experimental opportunities arise if epigenetic factors are considered as part of the molecular control of phenotype. Epigenetic mechanisms may explain paradoxical findings in twin and inbred animal studies when phenotypic differences occur in the absence of observable environmental differences and also when environmental differences do not significantly increase the degree of phenotypic variation.
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