In the version of this Letter originally published, an author error led to the affiliations for Brendan Payne, Jonathan Coxhead and Gavin Hudson being incorrect. The correct affiliations are: Brendan Payne: 3Wellcome Trust Centre for Mitochondrial Research, Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, UK. 6Institute of Neuroscience, Newcastle University, Newcastle upon Tyne, UK; this is a new affiliation 6 and subsequent existing affiliations have been renumbered. Jonathan Coxhead: 11Genomic Core Facility, Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, UK; this is a new affiliation 11 and subsequent existing affiliations have been renumbered. Gavin Hudson: 3Wellcome Trust Centre for Mitochondrial Research, Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, UK. In addition, in Fig. 2d, the numbers on the x-axis of the left plot were incorrectly labelled as negative; they should have been positive. These errors have now been corrected in all online versions of the Letter.