July 12, 2005 | David F. Coppedge

How Identical Are Identical Twins?

Identical twins look identical, and the assumption is that their genes are, too.  Not necessarily, found a team of European scientists publishing in PNAS.1  Their studies of genes from identical twins found that even when indistinguishable at birth, divergence over time in the expression of genes became evidence due to epigenetic (above-gene) factors:

MZ [monozygotic] twins constitute an excellent example of how genetically identical individuals can exhibit differences and therefore provide a unique model to study the contribution/role of epigenetic modifications in the establishment of the phenotype [i.e., physical appearance].  What does make MZ twins differ?  By using whole-genome and locus-specific approaches, we found that approximately one-third of MZ twins harbored epigenetic differences in DNA methylation and histone modification.  These differential markers between twins are distributed throughout their genomes, affecting repeat DNA sequences and single-copy genes, and have an important impact on gene expression.  We also established that these epigenetic markers were more distinct in MZ twins who were older, had different lifestyles, and had spent less of their lives together, underlining the significant role of environmental factors in translating a common genotype into a different phenotype.  Our findings also support the role of epigenetic differences in the discordant frequency/onset of diseases in MZ twins.

The differences could be due to drift over time by chance, or lifestyle choices, or differences in environment.  Their results, they concluded, did not prove heredity or environment were the decisive factors: “Our comparison of MZ twins suggests that external and/or internal factors can have an impact in the phenotype by altering the pattern of epigenetic modifications and thus modulating the genetic information.”

1Fraga et al., “Epigenetic differences arise during the lifetime of monozygotic twins,” Proceedings of the National Academy of Sciences, published online before print July 11, 2005, 10.1073/pnas.0500398102.

Philosophers have argued about heredity and environment for a long time.  These studies carry the debate down to the molecular level of DNA and gene expression.  No conclusions are drawn in this commentary, except the caution that creationists and evolutionists both need to take epigenetic factors into account before propounding simple explanations of stasis or evolution.  Genes are not the only things in control; a host of epigenetic factors contributes to the way a plant, animal, or human looks and acts.  Staging the debate as heredity vs environment begs the question of whether those are the only explanations for why identical twins diverge over their lifetimes.  It would seem that personal choices and unseen mental, moral or spiritual factors cannot be ruled out as significant agents of change that can extend down to the genes and influence their expression.

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