Abstract
Telomere length (TL) in humans is highly heritable and undergoes progressive age-dependent shortening in somatic cells. By contrast, sperm donated by older men display comparatively long telomeres, presumably because in the male germline, telomeres become longer with age. This puzzling phenomenon might explain why TL in the offspring correlates positively with paternal age. The present communication proposes that mitochondrial DNA polymorphisms and heteroplasmy cause variation in the production of reactive oxygen species, which, in turn, mediate age-dependent selection of germ stem cells with long telomeres and hence sperm with long telomeres. These long telomeres are then inherited by the offspring. The effect of paternal age on the offspring TL might be an evolutionarily driven mechanism that helps regulate TL across the human population.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.