They both had time to laugh before the signal died again. Charlie had been one of the most ardent advocates of Olivia turning her thesis into a book and she knew that he was really ringing to cheer her on. They had spoken a few days earlier about a challenging new paper she was determined to include, even at this late stage. Trying to find the right place for it was keeping her editing fresh. The experiment consisted of administering electric shocks to mice each time they were exposed to the sweet orange smell of acetophenone. It was hardly surprising that the mice, electrocuted five times a day for three days, started to become ‘reliably fearful’ from exposure to the smell alone. What was challenging to the standard view of inheritance was that the offspring of those mice still exhibited fear in the presence of acetophenone, without being given any electric shocks themselves. Indeed, the effect lasted into a third generation. The mothers of the new generations were of course unexposed, in case they passed on their fear in utero, and a further control was introduced through artificial insemination, eliminating the risk of rumours spreading from mouse to mouse through other forms of communication. Nobody understood how the fear of acetophenone had produced chemical changes that altered the expression of genes in a way that entered the sperm of the electrocuted mice. The orthodoxy of random mutation claimed that an organism’s complement of DNA was fixed from the moment of its conception. In this view it was impossible for a learnt aversive response to write itself into the DNA and be stably inherited by the offspring. It was just this kind of challenge to the conventional view of inheritance that made epigenetics such an exhilarating field.

Olivia looked up at the display of the stations that lay ahead. There were still three stops to Horsham. She knew she didn’t have the concentration to read any more but promised herself that during her stay with Francis, or at the very latest on the return journey on Sunday, she would find a place in her pages for the subversive, paradigm-challenging, reliably fearful mice. What she really needed to do was to get any texts and emails out of the way before she became absorbed in her torrid weekend. She ran through the new messages with the efficiency that could only come from deciding to postpone all the complicated ones, delete the intrusive ones and deal exclusively with Lucy’s confirmation of their dinner on Tuesday with a simple, ‘Can’t wait. Xx’.