Identical Twins: Are They Really Identical?

Nancy L. Pedersen, Ph.D., and David B. Allison, Ph.D.
University of Southern California and University of Alabama at Birmingham
NIEHS Grants R01ES010758 and R01ES009912

In groundbreaking research, an American, Swedish, and Dutch scientific team, including NIEHS-funded investigators, has shown that identical or monozygotic twins are indeed not genetically identical as was previously thought. This surprising finding may be of great significance for research on hereditary diseases and disorders and for the development of new diagnostic methods and treatments. Until now, environmental factors were thought to be the reason that one twin might develop an illness such as Parkinson’s disease while the other remained healthy. The current study suggests that other factors may be in play.

The team studies 19 pairs of monozygotic twins and found small subtle differences in their DNA due to copy number variation or CNV. CNV occurs when segments of DNA are missing or when extra copies of segments are inserted in the DNA sequence. Research suggests that CNV may play a bigger role in health and disease than previously thought. One recent study suggests sequence variation involving CNV is actually more important than single nucleotide polymorphisms.

DNA differences due to CNV may tell researchers if a missing gene or multiple copies of a gene are implicated in the onset of a disease. If one twin becomes develops a disease and the other doesn’t, the region of their genetic sequences that are different, becomes a target for further investigation to discover the basic genetic underpinnings of the disease.

Citation: Bruder CE, Piotrowski A, Gijsbers AA, Andersson R, Erickson S, de Ståhl TD, Menzel U, Sandgren J, von Tell D, Poplawski A, Crowley M, Crasto C, Partridge EC, Tiwari H, Allison DB, Komorowski J, van Ommen GJ, Boomsma DI, Pedersen NL, den Dunnen JT, Wirdefeldt K, Dumanski JP. Phenotypically concordant and discordant monozygotic twins display different DNA copy-number-variation profiles. Am J Hum Genet. 2008 Mar;82(3):763-71.