What traits are unique to each person?

The blood group is not unique – during organ transplants and blood transfusions, it is not common, but people who are identical in antigens are found. The handwriting, of course, is different for everyone, but it can be easily forged, and the same person can write in different handwritings. The voice depends on the length of the vocal cords and can also change throughout life – which means it is not unique. But the prints are the same throughout life, each person has his own. That is why they are used to identify criminals.