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Excerpt
It is easy for the novice in genetics to begin to think that biology is destiny. This study of identical twins is a reminder that there are many things in the environment that may modify the expression of a gene. I am sure you recall that identical twins are born with an identical set of genes. With that in mind, eighty Caucasian twins (40 pairs) were recruited for the study, including 30 male (15 pairs) and 50 female (25 pairs) participants. The youngest twins were 3 years old, and the oldest twins were 74 years old. Blood samples were analyzed to compare similarities and differences in the twins. Questionnaires were also completed about health, nutrition, physical activity, medication use, and tobacco/alcohol/drug consumption.
Results indicated that the youngest pair of twins had the most identical genomes. The oldest pairs of twins were, genetically speaking, the least alike. The older twins who were most different had different lifestyles and had spent less of their lives together. This did not mean that the genes had mutated or changed, but that methylation, a process of altering the expression of a gene without changing the gene’s DNA sequence, had occurred. Why? One theory is that external factors such as smoking, physical activity, or diet can have an influence on the expression of a gene. Another theory is that the aging process may be associated with “epigenetic drift,” i.e., as people age there may be small defects that affect the transmission of genetic information.
What are the implications? Chemical exposures, dietary habits, and environmental factors may all alter gene expression. Studies like this help us better understand how even genetically identical twins may be differentially susceptible to certain diseases. Most of all, studies like this one remind us that a gene’s DNA is not the whole story. Many external and internal factors may alter gene expression and the modulation of genetic information.
