Oxbridge Medicine Interview: A Genetics Question

Deyaanjali D. · Medicine (Oriel College, Oxford)

Hi, I'm Deya, a second-year medical student at Oriel College, Oxford. I want to walk you through a real example of the kind of question you can expect at an Oxbridge medicine interview, and, more importantly, how to think your way through it when you don't immediately know the answer. The question I'll use is a genetics one, and it's a lovely illustration of how these interviews actually work, because you can't answer it by remembering a fact. You have to reason.

The question: how is this disease inherited?

You're shown a pedigree chart of the inheritance of a genetic disease across three generations, and asked to explain how the disease is inherited. That's it. There's no formula to recall and no single right fact to blurt out. The interviewers want to watch you reason, so the very first thing to do is slow down and start thinking out loud.

First tip: say your options out loud

My first tip is to say what you're thinking as you think it, and to start from what you know. What modes of inheritance are there? You've got autosomal conditions and X-linked conditions, and each can be dominant or recessive. Simply listing those options does two things at once: it shows the interviewer you have a framework, and it gives you a structure to test the chart against, rather than guessing blindly.

Ruling out X-linked inheritance

Because there are affected females in the chart, the condition is unlikely to be X-linked recessive. Females have two copies of the X chromosome, so an affected female would need two mutated copies, and she'd always receive one normal copy from an unaffected father. It's also unlikely to be X-linked dominant, because a son who received only a mutated copy of the X chromosome would, in many such conditions, not be compatible with life. So before doing any probability at all, careful reasoning lets me set X-linked inheritance aside.

Testing autosomal inheritance against the pedigree

Next I'd consider autosomal inheritance. If a condition is autosomal recessive, there's a 25% chance of a child being affected. If it's autosomal dominant, there's a 50% chance, assuming the unaffected father is homozygous for the normal allele and not a carrier. A genuinely helpful move here is to sketch little Punnett-square-style plots and show them to the interviewer, so they can see how you work through the probabilities of a carrier versus an affected person. It turns an abstract answer into something you can point at.

Looking at the second generation, autosomal inheritance seems possible. But the third generation shows a clear pattern of unaffected and affected clusters of offspring that doesn't fit. The affected males have completely unaffected children, while the affected females have affected children. That's unusual for autosomal inheritance, where you'd expect that steady 25% or 50% probability regardless of the parent's sex.

The moment you might get stuck

So the condition is passed on by females only, both sexes can be affected, and it isn't X-linked. It's unlikely to be autosomal either. What other mode of inheritance is there? This is exactly the point where you might get stuck, and that's completely fine. The question is difficult because it forces you to take what you know and apply it in an unfamiliar context. The interviewers know that. They're there to help you through and give you support where you get stuck, so the worst thing you can do is panic, freeze and stop talking.

The breakthrough: mitochondrial inheritance

The way out is to keep reasoning aloud about what other kinds of genetic inheritance exist. Autosomal and X-linked inheritance are both chromosomal, and chromosomal inheritance involves nuclear DNA. So where else in the cell is DNA found? The mitochondria. Mitochondrial DNA is passed on to offspring only through maternal inheritance, because during fertilisation the sperm contributes its nuclear DNA to the egg but not its mitochondria. For this pedigree, where the condition travels strictly down the female line, mitochondrial inheritance is the most likely answer. Notice that the breakthrough didn't come from a memorised fact, it came from asking one more question when the obvious answers had run out.

What the interviewers are really looking for

This is the heart of it. The interview isn't a test of recall, it's a test of how you think when you're stretched. A few things genuinely earn credit:

• thinking out loud so the interviewer can follow your reasoning
• working from a clear framework and eliminating options with evidence from the chart
• drawing diagrams to externalise your thinking
• staying calm and carrying on when you hit the edge of what you know

You will inevitably come across hurdles, because the questions are designed to stretch you and find the limits of your knowledge, and to see how you cope with the unknown. That's the whole point.

Common mistakes to avoid

The pitfalls are all avoidable. Don't go silent when you're unsure, because silence tells the interviewer nothing. Don't jump to the first mode of inheritance you think of without checking it against every generation of the chart. And don't treat the interviewers as adversaries, they're closer to tutors running a demonstration lesson, so ask questions and tell them when you're stuck. Think of it as a demo tutorial, and don't be afraid to say when you're unsure.

Final thoughts

These are the sorts of strategies that helped me prepare for my own interviews and work through each question on the spot. Start from a framework, eliminate options with evidence, draw your thinking out, and when you hit a wall, keep reasoning aloud, because the answer is often one more question away, like remembering that DNA also lives in the mitochondria. It can feel daunting at first, but try to enjoy the chance to have a real discussion with experts in your field. If you'd like more support preparing for Oxbridge medicine interviews, Oxbridge Solution is here to help.