A Closed Loop

Here is a remarkable fact about identical twins: they have the same DNA, and therefore the same ‘genetic fingerprint’, yet their actual fingerprints (such as they might leave behind on a murder weapon) are different, and can be told apart in standard police observations. Fingerprints are, of course, produced by the pattern of tiny ridges in skin. So, it would appear that certain fine-scale details of our anatomy cannot be determined by a precise ‘genetic blueprint’.

It isn’t only fine details that seem open to negotiation in this way: anyone who has seen Bonsai cultivation knows how the very genes that would normally build a large tree can instead build a miniature-scale model, given a suitable environment. Bonsai trees aren’t completely scaled down, of course: their cells are normal-sized – it’s just that each component is made with fewer of them.

In the 1950 and ’60s, many children were affected by their mothers taking the drug thalidomide while pregnant, when the drug blocked growth of the internal parts of their limbs. Even though growth of the skin is...