Strange but ultimately positive genetic discovery reveals up to 1,000 relatives linked to prolific sperm donor

‘I suddenly discovered that I was part of one of the largest known single ancestor cohorts on planet Earth’: Jack Nunn with his mother Barbara. Photograph: Christopher Hopkins/The Guardian

Jack Nunn was 21, his girlfriend a year younger, when she died suddenly while the pair were in England.

Nunn had been studying literature, but that shocking tragedy in 2007 threw him on to a new path that would end in a strange but unexpectedly positive discovery – that his grandfather was one of the world’s most prolific sperm donors, leaving him with thousands of close relatives.