Variation
How can mutations cause disease?
Sometimes, one of the DNA letters is accidentally swapped for another letter. This mutation might have a very serious effect, or none at all.
If the letter T is swapped for an A in the codon GCT then the protein will still be the same, since both the old codon (GCT) and the new codon (GCA) code for the amino acid Alanine.
However, swapping an A for a T in a gene for haemoglobin causes a serious disease called sickle cell anaemia. Haemoglobin does the job of carrying oxygen around the body in the blood. In people with sickle cell anaemia, the sequence of amino acids in haemoglobin is different, and so it doesn't work as well.
There are other types of mutations as well. Sometimes, a bit of the DNA sequence is missed out by mistake, or a new bit added in. Sometimes, parts of the sequence are swapped over, even between different chromosomes.
Inheriting mutations
Each of our genes is a copy from either our mum or our dad. If there is a mutation in one of these genes, this can be passed on from parent to child along with the rest of the gene. This is why diseases can run in families.
Small inherited changes can make big differences in our bodies. For example, the most common mutation to cause cystic fibrosis - a disorder where a person's internal organs become clogged with thick mucus - is the loss of three letters in a gene called CFTR.
