What happens in people that have this difference in their DNA?

Explanation:

DNA comes in long strands called chromosomes, which are kept inside the nucleus of a cell. Every one of our cells has a nucleus with all of the DNA. Each cell is different not because of the bits of DNA in it, but because of the bits it uses to make things.

Each strand of DNA is made up by joining together nucleotides. There are four different nucleotides called adenine (A), thymine (T), cytosine (C) and guanine (G). If you could see the nucleotides in a strand of DNA it would look something like this:

It is quite extraordinary that just four different nucleotides make up the code of every single living thing. The order that these nucleotides are in determines the difference between a brain cell, a toenail, a blood cell and whether or not we have a certain disease.

So if you wrote out the DNA sequences of two people and matched them up together, they would look almost exactly the same. There would be about 1 difference in every 100 nucleotides (it’s actually more like 1 in 1000). These differences determine whether you have blonde hair or brown, whether you are tall or short, whether you will have asthma or not.

You are more similar to people you are related to because your DNA came from the same place; your great, great, great grandparents, or something like that. Your DNA is a combination of your Mum and Dad’s DNA, half from each. So you are half the same as your Mum, half the same as your Dad. If you looked at the DNA of a friend you are not related to, they would also be half the same as their Mum, and half the same as their Dad, but they would be quite different to you. This is because the last common ancestor (person who is related to you both) was so many hundreds or thousands of years ago, that a lot of changes have occurred in the DNA sequence since then.