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How many genetic differences are there from one person to another?

Michel Cameron, PhD
Michel Cameron, PhD
Pharmacogenomics Director

To answer this fascinating question, which I am often asked, I should first explain that the human genetic code is a type of chain formed by links called nucleotides. There are four different nucleotides: adenine, thymine, cytosine and guanine, currently represented by the letters A, T, C and G.

It is estimated that our DNA sequence—the nucleotide chain that makes up our personal genetic code—consists of about 3 billion nucleotide pairs (nucleotides always come in pairs: A always goes with T, and C goes with G). But since our cells contain two copies of DNA code (one from our father, one from our mother), a person’s DNA code is actually made up of about 6 billion nucleotide pairs.

And that’s where it gets fascinating! When we compare the DNA codes of two different people, they are practically the same. In fact, they are 99.9% identical. But since the DNA code is so long, this 0.1% difference is equivalent to about 6 million genetic differences between two individuals!

So this 0.1% is the reason why some people are more susceptible to a particular disease or more likely to be in good health than their neighbour—or even another family member.

This genetic variation also explains physical differences between people, such as eye colour and blood type. It also explains why some people inherit relatively rare conditions like cystic fibrosis and muscular dystrophy, or why they inherit an increased risk of developing more common diseases like cancer, heart disease and asthma.

Understanding how this 0.1% variation in human genetics influences health and disease is a top priority of medical science...

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Michel Cameron, PhD
Michel Cameron, PhD
Pharmacogenomics Director