Paul Kammerer

Although I am not a genetics expert, the Science news article "The Case of the Midwife Toad: Fraud or Epigenetics? piqued my interest. It would be interesting to see if someone finally manages to repeat Paul Kammerer's experiment.

Constraints and Restraints

In the paper "A short history of SHELX" by George Sheldrick (Acta Crystallographica A, 2008, 64, 112-122) is explained very clearly something I did not completely realise:

- CONstraints lower the number of variables to refine, while

- REstraints augment the number of observations.

What follows from this is that for low resolution structures introducing extra constraints (for instance strict NCS) is more efficient than introducing extra restraints, the reason being that you need more than one observation per refined variable.

This is not actually implemented in all crystallographic protein structure refinement programs, which I think is a shame.