FORENSIC IMPLICATIONS OF THECIRCULATORY SYSTEM

• Blood found at a crime scene can be used to identify the victim or the perpetrator.

• Blood type is a class characteristic. – Ex. if the victim had O blood,

finding AB blood at the crime scene suggests that the perpetrator had that blood type. Many people may have that blood type, but suspects with any other blood type can be excluded.

FORENSIC IMPLICATIONS OF THECIRCULATORY SYSTEM

• Nuclear DNA found in the blood can be used as individual evidence. – Red blood cells do not have a

nucleus or DNA. – However, white blood cells have

a nucleus and nuclear DNA. – Except for identical twins, each

person’s nuclear DNA is unique. • Therefore, DNA can be used to

identify the victim or the perpetrator.

Blood

Blood is biological evidence—it comes from a living or once-living source. • When blood evidence is found at a crime

scene, investigators ensure the integrity of the evidence through proper collection and evaluation techniques.

Red Stuff

• when a red stain is found, investigators must ask three basic questions:– Is it blood?– Is it human blood?– If it is human blood, can the blood be traced to a

single person?

Hemastix® (for spot stains)• presumptive test used to

indicate whether the red stain might be blood (Figure 6-8).

• plastic strips that have been treated with a special blood reagent. – If the stain is blood, the

reagent on the test strip turns green. (Iron in hemoglobin acts as a catalyst in this reaction)• A catalyst is any substance that

speeds up a chemical reaction.

Luminol (for large area of blood):• presumptive test—• Mixed with hydrogen

peroxide.• During the reaction with

hydrogen peroxide, the luminol is oxidized and light energy is released (see Figure 6-9).– Therefore, the luminol test is

viewed in a darkened area.

The Leucomalachite green • Presumptive test • based on the same

reaction in luminol but in the presence of iron:– Leucomalachite green

turns blue-green.

Kastle-Meyer test. • Presumptive test• A solution of

phenolphthalein is used, which turns pink in the presence of traces of blood.

Collection of blood evidence:• Bag each evidence in a

paper bag separately to avoid cross-contamination of the evidence.

• If the blood evidence is found on a small object, such as a pencil or soda can, the object is packaged and the blood evidence is removed at the lab.

Collection of blood evidence:• If the blood evidence is found on a

large object, such as a door or wall, only the blood evidence is collected. – Investigators collect dried

blood in a variety of ways. • wet swab to remove the

blood. • place fingerprint tape over the

blood and lift the stain,• use a sharp instrument to

scrape blood into a paper bag.

Collection of Evidence

Note: • Blood needs to be air dried. – Drying the blood prevents mold or other

microorganisms from forming. • Microorganisms can destroy the evidence.

• Evidence collected from the suspect acts as a control or known sample.