dna/blood

[jannyneve]

if 4 related people have the same blood group and one commited a crime and left dna evidence at the scene would it be possible to detect for certain which one did it?

[OBonio]

Yes, DNA is unique to each and every one of us. Blood group isn't.

[morg_monster]

DNA is unique however when the police do a DNA test they only check a few 'bits' of DNA, which are more likely to be the same in related persons. So the police would need to do a more comprehensive DNA test to differentiate between the relatives.

(Similar to if paternity of a child is in doubt and the two possible candidates are brothers).

[boobesque]

planning anything big?

[bernardo]

Yes - except for identical twins (or triplets (or quadruplets (or quintuplets (etc)))). If they caught one of the relatives and thought he was a suspect, they would be able to tell that he was related to the criminal because the DNA would be similar. The DNA of identical twins is identical, but the fingerprints are not - so if an identical twin committed a crime and left traces of DNA and fingerprints at the crime scene, the police would be able to tell which one did it. If there were no fingerprints or other evidence, they wouldn't be sure.