Fig. 1

Concept of transgenerational telomere erosion in the female germline (as published in [9, 15]): High expression levels of telomerase stabilize sperm telomeres in human males throughout reproductive life (flat red line). Already at prenatal age, the ovaries are populated with millions of primary oocytes during oocytogenesis. Only a small fraction of them complete meiosis and start ovulating in a serial manner at menarche. It has been postulated that long-telomered ova are utilized first [16], whereas female germ cells with critically short telomeres are last (descending blue line). Support for this hypothesis comes from the well-known fact that middle-aged women have an increased risk of aneuploid pregnancies (e.g. Down’s syndrome), which rapidly increases during the second half of the female reproductive period [17]. In diploid organisms, at conception, one chromosome comes from the mother the other from the father, resulting in a combination of parental chromosomes and thereby telomere lengths in the fertilized egg (junctions of red & blue lines). Older males bypass the telomere loss of 1–2 generations and on average have longer sperm telomeres than their younger contemporaries. Several scenarios and combinations are highlighted in the graph (colored broken lines). The negative maternal age effect on offspring’s telomeres is masked by the fact that older women tend to have male partners of similar advanced age (blue broken line). The positive paternal age effect is pronounced, because the reproductive period of males is twice as long as in females and therefore old fathers can bypass the telomere loss of two generations (yellow and green broken lines)