As Rosie picked up Belinda’s cup, Belinda said, ‘Leave it, I’ll do it later.’

Rosie responded, ‘No, please,’ and took Eamonn’s cup.

Belinda picked up my cup and Rosie’s and said, ‘Okay, give me a hand.’ They walked out to the kitchen together. It was obviously going to be difficult for Rosie to swab Eamonn’s cup with Belinda present, but I could not think of a way of getting Belinda out of the kitchen.

‘Did Rosie tell you I studied medicine with her mother?’ asked Eamonn.

I nodded. Had I been a psychologist, I might have been able to infer from Eamonn’s conversation and body language whether he was hiding the fact that he was Rosie’s father. I might even have been able to lead the conversation in a direction to trap him. Fortunately we were not relying on my skills in this arena. If Rosie succeeded in collecting the sample, I would be able to provide a far more reliable answer than one derived from observations of behaviour.

‘If I can offer you a little encouragement,’ Eamonn said, ‘Rosie’s mother was a bit wild in her younger days. Very smart, good-looking, she could have had anyone. All the other women in medicine were going to marry doctors.’ He smiled. ‘But she surprised us all and picked the guy from left field who persisted and stuck around.’

It was lucky I wasn’t looking for clues. My expression must have conveyed my total lack of comprehension.

‘I suspect Rosie may follow in her mother’s footsteps,’ he said.

‘In what component of her life?’ It seemed safer to seek clarification than assume that he meant getting pregnant to an unknown fellow student or dying. These were the only facts I knew about Rosie’s mother.

‘I’m just saying I think you’re probably good for her. And she’s had a rough time. Tell me to mind my own business if you like. But she’s a great kid.’

Now the intent of the conversation was clear, although Rosie was surely too old to be referred to as a kid. Eamonn thought I was Rosie’s boyfriend. It was an understandable error. Correcting it would necessarily involve telling a lie, so I decided to remain silent. Then we heard the sound of breaking crockery.

Eamonn called out, ‘Everything okay?’

‘Just broke a cup,’ said Belinda.

Breaking the cup was not part of the plan. Presumably, Rosie had dropped it in her nervousness or in trying to keep it from Belinda. I was annoyed at myself for not having a back-up plan. I had not treated this project as serious field work. It was embarrassingly unprofessional, and it was now my responsibility to find a solution. It would surely involve deception, and I am not skilled at deception.

My best approach was to source the DNA for a legitimate reason.

‘Have you heard about the Genographic Project?’

‘No,’ said Eamonn.

I explained that with a sample of his DNA we could trace his distant ancestry. He was fascinated. I offered to have his DNA processed if he organised a cheek scraping and sent it to me.

‘Let’s do it now, before I forget,’ he said. ‘Will blood do?’

‘Blood is ideal for DNA testing, but –’

‘I’m a doctor,’ he said. ‘Give me a minute.’

Eamonn left the room, and I could hear Belinda and Rosie speaking in the kitchen.

Belinda said, ‘Seen your father at all?’

‘Next question,’ said Rosie.

Belinda instead responded with a statement. ‘Don seems nice.’

Excellent. I was doing well.

‘Just a friend,’ said Rosie.

If she knew how many friends I had, she might have realised what a great compliment she had paid me.

‘Oh well,’ said Belinda.

Rosie and Belinda returned to the living room at the same time as Eamonn with his doctor’s bag. Belinda reasonably deduced that there was some medical problem, but Eamonn explained about the Genographic Project. Belinda was a nurse and she took the blood with professional expertise.

As I handed the filled tube to Rosie to put in her handbag, I noticed her hands were shaking. I diagnosed anxiety, presumably related to the imminent confirmation of her paternity. I was not surprised when she asked, only seconds after leaving the Hughes’s residence, if we could process the DNA sample immediately. It would require opening the lab on a Saturday evening but at least the project would be completed.

The laboratory was empty: throughout the university, the archaic idea of working Monday to Friday results in an incredible under-utilisation of expensive facilities. The university was trialling analysis equipment that could test for parent–child relationships very quickly. And we had an ideal DNA sample. It is possible to extract DNA from a wide variety of sources and only a few cells are needed for an analysis, but the preparatory work can be time consuming and complex. Blood was easy.

The new machine was located in a small room that had once been a tea-room with sink and refrigerator. For a moment I wished it had been more impressive – an unusual intrusion of ego into my thoughts. I unlocked the refrigerator and opened a beer. Rosie coughed loudly. I recognised the code and opened one for her also.

I tried to explain the process to Rosie as I set up, but she seemed unable to stop talking, even as she used the scraper on her inner cheek to provide me with her DNA sample.

‘I can’t believe it’s this easy. This quick. I think I’ve always known at some level. He used to bring me stuff when I was a kid.’

‘It’s a vastly over-specified machine for such a trivial task.’