Over the next few articles, we’ll explore a little about genetics, our family history, and cancer risk.
First, we need to cover some of the science behind how genetics actually works.
Imagine a book. The pages have that fresh, just-printed smell.
Inside, there are 46 chapters. Within each chapter are thousands of sentences, made up of words, which are built from letters.
Now picture this book sitting on a bookshelf, alongside many others.
With enough shelves, you have a library. All the books in this library use the same letters, but the way those letters are arranged makes every book unique. In fact, 99.9% of the words are the same from one book to another — it’s that tiny 0.1% that makes each story different.
The books are grouped by genre. Some shelves hold cookbooks, others thrillers, or romance novels.
They all use similar words, but the arrangement and focus change the type of story they tell.
Now imagine the library is busy.
One book is very popular. It’s picked up and put down all the time.
The pages get worn at the edges. Someone spills their lunch on it, smudging some of the most important sentences. Maybe a page is torn out, or water from a leaking ceiling stains the paper. Over time, the story becomes harder to follow. Eventually, the book might have to be taken off the shelf.
On the shelf above, another book sits more peacefully. Readers are careful.
They use bookmarks, read it under good lighting, and maybe even wrap it in a dust jacket. That book will last much longer.
The books are us. Each of the 46 chapters is a chromosome. Inside each chapter are sentences — your genes. Each sentence is made up of words — your DNA. And those words are built from just four letters, called nucleotides.
We are born with our own special book. But as we move through life, the way that book is handled changes it. Sunburns, infections, pollution, smoking — they can leave smudges or rips in the pages. Some changes are small. Others can damage the story so much that the body gets confused — and that’s where diseases like cancer can begin.
But we’re not helpless. In previous articles, we’ve explored ways to protect our book: through exercise, healthy food, sleep, and care. You can’t rewrite the book, but you can protect the pages.
In life, two exact copies of the book are called identical twins. The books are the same at first.
But over time, the environment they’re kept in — and how carefully they’re looked after — means they slowly become different.
This helps explain how one twin might develop a condition the other doesn’t, even though genetically, they start out the same.
Life on Earth is made of DNA — the same four letters (A,T,C,G), telling billions of different stories.
Given our complicated human history, it may be surprising to know that you share 99.9% of your DNA with every other human on Earth. It’s that tiny 0.1% that makes each of us unique — including the genes that influence everything from eye colour to cancer risk.
Humans and chimpanzees share about 98.8% of their DNA. It’s the remaining 1.2% that makes us wear trousers and style our hair. And next time you’re in the fruit section of the supermarket, look at the bananas — you share about 60% of your DNA with that little yellow fruit.
Many people have taken genetic tests to learn more about their ancestry or race. But here’s the truth: Race has nothing to do with genetics.
Race is complex. It might involve your passport, your neighbourhood, your cultural identity, and sometimes, other people’s assumptions. But it’s not written in your DNA.
Ancestry can include some genetic clues — certain genes may be more common in people from certain regions — but there’s no “Irish gene” or “African gene” or “Asian gene.”
By 1900, Ireland’s population had halved compared to pre-famine years. Even now, nearly 200 years later, it remains the only European country whose population has still not fully recovered to its pre-famine size. Our heavy reliance on a single type of potato meant that when blight struck, there was no genetic diversity to resist it — and no way for the crop to survive.
Diversity doesn’t just apply to plants — it’s crucial for human health, too. Mixing genes is crucial for a species’ survival, whether you’re a potato, a dog breed, or a person. It’s one of the key reasons why marrying your cousin isn’t such a good idea.
I have a strong interest in Irish history and deeply admire many of our Irish patriots. But the truth is, here in 2025, I likely have more in common with the people I work with every day than with those who lived two centuries ago. Many of my work colleagues were born thousands of miles from Cork — yet now we collectively grapple with the same, very modern problems, like when to give our kids a phone, or which energy provider offers the best deal.
If you have European ancestry, you’re almost certainly related to Julius Caesar — not because he had many children, but because people from his time who did have children are now ancestors of nearly everyone in Europe. After 2,000 years, family trees overlap a lot.
Yes, that also means you’re probably descended from royalty in the genealogical sense — but your chances of inheriting a throne anytime soon? Not a chance.
Next time, we’ll look at the role of genetics in cancer.
Know the facts — own your risk — decide for yourself.
