Blood

Use of Blood

Provides class evidence because more than one person has the same blood type

White blood cells contain DNA so it’s possible to determine if blood at a crime scene is from a suspect or a victim

Composition of Blood

Plasma – the straw-colored liquid portion of the blood

Red Blood Cells – function in carrying oxygen and carbon dioxide

White Blood Cells – function in the immune system; only blood cell to contain DNA

Platelets – function in clotting

DNA Profiling

DNA evidence has been used by the Innocence Project to help free inmates falsely convicted of crimes

Blood Typing

Less expensive way to analyze blood evidence than DNA analysis

This is a form of class evidence because

it can only identify a group of individuals Different proteins are present on the

surface of red blood cells and these determine blood type

Blood Types

Type O – does not have any proteins on the surface of red blood cells (43% of people are Type O)

Type A – has protein A on the surface of red blood cells (42%)

Type B - has protein B on the surface of red blood cells (12%)

Type AB – has both protein A & B on the surface of red blood cells (3%)

Rh Factor

Another protein on the surface of red blood cells

85% of people have this protein (Rh+)

15% of people do not have this protein (Rh-)

Blood Typing Tests

There are three items to test for: A protein

B protein

Rh protein

There are many other blood proteins (hundreds) that can further help to identify an individual

Probability and Blood Types

We can calculate the percentage of the population who has a particular type of blood

Ex: What percentage of people have A+ blood?

Type A – 42%

Positive – 85%

Convert these to decimals, multiply them, then multiply by 100 to get a percentage

0.42 x 0.85 = 0.357 x 100 = 35.7%

36 out of 100 people should be A+

Universal Donor – O negative

Can be given to anyone because it doesn’t have any proteins on the surface of it’s red blood cells

Universal Recipient – AB positive

Can get any blood because it has all proteins on the surface of it’s red blood cells