For some time, Rosie did not mention the Father Project. I attributed this to the improved relationship with Phil and the distraction of other activities. But, in the background, I was processing some new information.

At the wedding, Dr Eamonn Hughes, the first person we had tested, asked to speak to me privately.

‘There’s something you should know,’ he said. ‘About Rosie’s father.’

It seemed entirely plausible that Rosie’s mother’s closest friend from medical school would know the answer. Perhaps we had only needed to ask. But Eamonn was referring to something else. He pointed to Phil.

‘Phil’s been a bit of a screw-up with Rosie.’

So it wasn’t only Rosie who thought Phil was a poor parent.

‘You know about the car accident?’

I nodded, although I had no detailed information. Rosie had made it clear that she did not want to discuss it.

‘Bernadette was driving because Phil had been drinking.’

I had deduced that Phil was in the car.

‘Phil got out, with a broken pelvis, and pulled Rosie out.’ Eamonn paused. He was obviously distressed. ‘He pulled Rosie out first.’

This was truly an awful scenario, but as a geneticist my immediate thought was ‘of course’. Phil’s behaviour, in pain and under extreme pressure, would surely have been instinctual. Such life-and-death situations occur regularly in the animal kingdom and Phil’s choice was in line with theory and experimental results. While he had presumably revisited that moment many times in his mind, and his later feelings towards Rosie may have been severely affected by it, his actions were consistent with the primitive drive to protect the carrier of his genes.

It was only later that I realised my obvious error. As Rosie was not Phil’s biological daughter, such instincts would not have been applicable. I spent some time reflecting on the possible explanations for his behaviour. I did not share my thoughts or the hypothesis I formed.

When I was established at Columbia, I requested permission to use the DNA-testing facilities for a private investigation. They were willing to let me do so. It would not have been a problem if they had refused. I could have sent my remaining samples to a commercial laboratory and paid a few hundred dollars for the tests. This option had been available to Rosie from the beginning of the Father Project. It is now obvious to me that I did not alert Rosie to that option because I was subconsciously interested in a relationship with her even then. Amazing!

I did not tell Rosie about the test. One day I just packed my bag with the samples that I had brought with me to New York.

I started with the paranoid plastic surgeon, Freyberg, who was the least likely candidate in my assessment. A green-eyed father was not impossible, but there was no other evidence making him more probable than any of the previous candidates. His reluctance to send me a blood sample was explained by him being a generally suspicious and unhelpful person. My prediction was correct.

I loaded Esler’s specimen, a swab from a fork that had travelled more than halfway around the world and back again. In his darkened basement, I had been certain he was Rosie’s father. But afterwards I had come to the conclusion that he could have been protecting a friend or the memory of a friend. I wondered if Esler’s decision to become a psychiatrist had been influenced by the suicide of the best man at his wedding, Geoffrey Case.

I tested the sample. Isaac Esler was not Rosie’s father.

I picked up Gene’s sample. My best friend. He was working hard on his marriage. The map was no longer on his wall when I went in to submit my resignation to the Dean. But I had no recollection of seeing a pin in Ireland, Rosie’s mother’s birthplace. There was no need to test the table napkin. I tossed it in the waste bin.

I had now eliminated every candidate except Geoffrey Case. Isaac Esler had told me that he knew who Rosie’s father was and that he was sworn to secrecy. Did Rosie’s mother – and Esler – not want Rosie to know that there was a family history of suicide? Or perhaps a genetic predisposition to mental illness? Or that Geoffrey Case had possibly killed himself in the wake of the news that he was Rosie’s father and that her mother had decided to remain with Phil? These were all good reasons – good enough that I considered it highly likely that Rosie’s mother’s one-night encounter had been with Geoffrey Case.

I reached into my bag and pulled out the DNA sample that fate had delivered to me without Rosie’s knowledge. I was now almost certain that it would confirm my hypothesis as to her paternity.

I cut a small portion of the cloth, poured over the reagent, and let it sit for a few minutes. As I watched the fabric in the clear solution, and mentally reviewed the Father Project, I became more and more confident in my prediction. I decided that Rosie should join me for this result, regardless of whether I was right or wrong. I texted her. She was on campus and arrived a few minutes later. She immediately realised what I was doing.

I put the processed sample in the machine, and waited while the analysis proceeded. We watched the computer screen together until the result came up. After all the blood-collecting, cheek-swabbing, cocktail-shaking, wall-climbing, glass-collecting, flying, driving, proposal-writing, urine-mopping, cup-stealing, fork-wiping, tissue-retrieving, toothbrush-stealing, hairbrush-cleaning and tear-wiping, we had a match.

Rosie had wanted to know who her biological father was. Her mother had wanted the identity of the man she had sex with, perhaps only once, on an occasion of emotion-driven rule-breaking, to remain a secret forever. I could now fulfil both of their wishes.