1. Say for example that you wanted to find out who was the father of your child. We provide you with a small brush and you scrape the inside of your cheek, the cheeks of the child and the other adults you want to test.

2. Then you post the brushes to our lab.

3. We take the DNA from each brush and increase the important 'marker' areas.

Then we put the DNA on a special gel (polyacrylamide) and send a large surge of electricity through it. All the markers separate out into their different sizes and then they can be checked.

 

4. All the bands in the child's DNA should match either the father or the mother's DNA. So, if you match a child's DNA with its mother (red), then what is left over has to come from the father (blue).

In this case all the child's DNA matches either the mother or the alleged father, so it is extremely likely that he is the true father.

If it doesn't match the alleged father, then some other man is the father (see below).

In this case though, there are a number of black bands that do not match either the mother or the possible father, so it is unlikely that the man is the father of the child.

In practice, many more markers are used than are shown in the diagram.