Past exam of the genetics course of the University of Cambridge / 2024 / Paper 1 / 7 / a / Solution

A useful schematic is the upper critical solution temperature form of a binodal: a dome in temperature versus chromatin concentration. Below the dome, an intermediate overall concentration separates into a dilute phase and a chromatin-rich phase; above it the liquid is homogeneous. At a fixed temperature, the two intersections of a horizontal tie line give the compositions of the coexisting phases, not the concentrations of two different initial samples.

The diagram is a model, not measured phase-boundary data. The stated observation at one salt concentration does not by itself establish whether heating suppresses or promotes demixing. A lower critical solution temperature system would instead demix on heating, and would need the corresponding inverted geometry. The following answers make the usual enthalpically favourable-demixing assumption explicit.