Why identical twins aren't

kw: commentary, genetics, randomness

Take a look at this picture, from the cover of Science (Aug 16, 2002). The caption states, "Bacterial cells simultaneously expressing two different fluorescent proteins (red and green) from identical promoters. Because of stochasticity ('noise') in the process of gene expression, even two nearly identical genes often produce unequal amounts of protein. The resulting color variation shows how noise fundamentally limits the accuracy of gene regulation."

The article on page 1183 of that issue, "Stochastic Gene Expression in a Single Cell" by Michael B. Elowitz, Arnold J. Levine, Eric D. Siggia, and Peter S. Swain, reports on their experiments that show how random noise in and around the cellular machinery for gene expression causes variations in expression even when the genome is identical, or nearly so. The cover photo and links to the article are found at http://www.sciencemag.org/content/vol297/issue5584/cover.dtl.

I was looking for this picture when I posted More Variety Than a Field of Snowflakes, about "No Two Alike" by Judy Rich Harris. It is known that, although identical twins share nearly identical genomes (differences are due to tiny amounts of random mutations during embryo development), they can be remarkably different; and that variations in most characteristics that can be measured are about 50% genetic, and 50% unknown. Ms Harris focuses on environmental factors. Based on the above article, I also considered inherent randomness. There are three principal sources of stochastic difference in the genetics and gene expression between two cells that result from a single cell dividing by mitosis:

1. Mutations that occur during DNA duplication (there are several varieties)
   
2. Random variations in the molecular buffeting that the gene expression machinery undergoes (Brownian motion)
   
3. Random variations in everything else going on in the cell at the time

All varieties of reaction to one's environment involve changes of gene expression. Also, any organism's behavior, determined by gene expression, will affect that environment; the environment reacts. Thus, any difference in expression will modify the environment, and its changes cause further reaction by the organism. This feedback may be positive (spiraling out of control) or negative (tending to a stable result little changed by the variation).

So why are twins different? Some randomness in reaction leads to positive feedback, increasing the difference between the twins.