1. Blood stains 4.10 What can you tell from a blood stain?

2. If someone is shot, stabbed, cut or beaten up there’s likely to be some blood at the scene of the crime. This blood usually comes from the victim but if the perpetrator also left some of their blood at the scene, the forensic scientists can tell you quite a lot about the villain from a single speck of red!

3. Human blood contains proteins on the surface of the red cells and floating freely in the plasma. These proteins can be used to sort the blood into groups. There are four major groups: A, B, AB and O. The group you’re in never changes.

4. Blood is also coded for other proteins – an example is the rhesus protein. You are rhesus positive if you have the rhesus protein and rhesus negative if you do not. So four groups become eight: A+, A–, B+, B–, AB+, AB–, O+ and O–. In fact there are many more blood proteins and even the most common combination of these different proteins only occurs in 1 in 256 people! If a villain leaves some blood at a crime the search can be narrowed down.

5. In fact, blood groups are usually used to rule people out of a crime investigation. If your group does not match the blood at the scene then the blood cannot be yours. However, if your blood group does match the criminal’s this does not prove that you committed the crime. Why not?  Northern Ireland Police Force

6. There is always a chance that two people will have the same blood group, even when you take all of the different proteins into account. To identify someone uniquely you need a different test – DNA fingerprinting. This is completely unique and scientists can create one from even the smallest blood stain.

7. The DNA in the blood cells is treated with enzymes which split it into fragments of different sizes. These pieces are separated using a weak electric field and at some point a special photograph is taken to show where all of the pieces have ended up. It’s a bit like chromatography of ink mixtures. The pattern is called a DNA fingerprint and computers can compare a criminal’s fingerprint with millions of others stored in a database. If the DNA fingerprints match, the computer can produce the person’s name and a range of other details.

8. A forensic scientist can find out so much from a blood sample that Scene of Crime Officers will look very carefully to find the smallest speck. Sometimes SOCOs will spray an area with fluorescin. This reacts with the iron in the blood to make a chemical that glows in ultraviolet light. This shows up the tiniest speck of blood.

9. Even the shape of a blood spot can be useful. Long thin splashes show that the blood has come from a long way and hit the ground at an angle. Round spots show the blood fell a short distance straight to the ground. By plotting the shape of the spots in a spray of blood you can work out where it came from to see if the victim was shot standing up or lying down.

10. So what can blood show? Which of these pieces of evidence could lead to a conviction in court? The blood group at a crime scene matches the group of the main suspect. The DNA fingerprint matches the main suspect. The blood type at a crime scene is A. The main suspect is group AB. Blood stains on the main suspect’s clothes match the group of the victim. The DNA fingerprint on a knife from the suspect’s house matches blood from the murder victim.