Same But Different

Subtitle:  How epigenetics can blur the line between nature and nurture.

We briefly discussed the nature vs nuture phenomenon with genes, and as I mentioned Siddartha Mukherjee, author of The Gene: An Intimate History, had a mother and aunt who were identical twins.  The article points out that in the late 1970s a team of scientists in Minnesota set out to determine why identical twins are similar, i.e., how much these similarities arose from genes, rather than environments—from “nature,” rather than “nurture.”  The question Mukherjee pursues is this:

Why are identical twins different? Because, you might answer, fate impinges differently on their bodies. One twin falls down the crumbling stairs of her Calcutta house and breaks her ankle; the other scalds her thigh on a tipped cup of coffee in a European station. Each acquires the wounds, calluses, and memories of chance and fate. But how are these changes recorded, so that they persist over the years? We know that the genome can manufacture identity; the trickier question is how it gives rise to difference.

The genome is not a passive blueprint.  When one twin breaks an ankle and acquires a gash in the skin, wound-healing and bone-repairing genes are turned on, thereby recording a scar in one body but not the other.  This article goes into some fascinating detail about how David Allis and many other scientists, figured out over decades that  protein systems, overlaying information on the genome, generated the bewildering intricacy necessary for a cell to build a constellation of other cells out of the same genes, and for the cells to add “memories” to their genomes and transmit these memories to their progeny. “There’s an epigenetic code, just like there’s a genetic code,” Allis said. “There are codes to make parts of the genome more active, and codes to make them inactive.”

So, why are twins different? Well, because if you sequence the genomes of a pair of identical twins every decade for fifty years, you get the same sequence over and over. But if you sequence the epigenomes of a pair of twins you find substantial differences: the pattern of epigenetic marks on the genomes of their various cells, virtually identical at the start of the experiment, diverges over time.

I know this is a long New Yorker article, but the science is fascinating for those interested in reading further, or get his book, or read a review about the book.