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BLOODSTAIN PATTERNS

BLOOD. BLOODSTAIN PATTERNS. WHAT TYPE ARE YOU?. Blood and bloodstains can help identify a suspect from a crime scene The three main factor involving forensic analysis of blood: ABO system, MNS system, and Rh factor. The ABO Blood Sytem.

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BLOODSTAIN PATTERNS

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  1. BLOOD BLOODSTAIN PATTERNS

  2. WHAT TYPE ARE YOU? • Blood and bloodstains can help identify a suspect from a crime scene • The three main factor involving forensic analysis of blood: ABO system, MNS system, and Rh factor.

  3. The ABO Blood Sytem • The ABO blood system is important in blood transfusions. • On the surface of red blood cells there are molecules called antigens, A and B are two different types of antigens. • There are four blood types: • A (just A antigens) • B (just B antigens) • AB (both A and B antigens) or • O (no antigens present on the cells surface.)

  4. The ABO Blood Sytem Cont... • Antibodies are produced by the immune system to destroy foreign substances. • Type O contains both anti-A and anti-B antibodies. • Those with Type A blood have anti-B antibodies to defend against B antigens. • These antibodies will attach to the antigens and clump.

  5. Antibodies and antigens

  6. Antibodies and antigens A antigens Incompatible! H H Anti-A antibodies Compatible! Anti-B antibodies Clumping ensues

  7. Testing For Blood Type • Testing a sample of blood with anti-A or anti-B serum will determine the type. • If the blood cells contains the antigens opposite the antibodies in the serum, it will clump. • This is why blood transfusions only work with the proper blood.

  8. The Inheritence of Blood Type • A Punnett square can show how different alleles are inherited to form a blood type. • Each parent contributes one allele. • If both parents are homozygous for the same blood type, then all of their children will have the same phenotype and genotype. • O antigens are recessive, ergo the child must receive one recessive O allele from each parent to have O blood type. • AO (or BO) will have A blood type (or B) but can pass an O allele to their offspring.

  9. Punnett squares are fun! Mom: A A Dad: A A AA AA Kids all have type A blood AA AA

  10. Punnett squares are fun! Mom: A B Dad: A B AA AB 50% of kids are AB 25% are A 25% are B BB AB

  11. Punnett squares are fun! Mom: A o Dad: B o 25% of kids are AB 25% are A (but carry O) 25% are B (but carry O) 25% are O AB Bo oo Ao

  12. Rh Factor • Rh Positive or Rh Negative • Rh positive have the Rh antigen on the blood cells and Rh negative do not. • A Rh+ transfusion to an Rh- patient can cause major problems.

  13. The MNS Blood System • Individuals carry one or two different molecules on the surface of red blood cells. • These molecules are antigens, but the human body does not produce the antibodies for them so they are not medically important to humans. • People with group M have one of these molecules, while people with group N have a different antigen molecule. • The MN group is characterized by the presence of both molecules and so on.

  14. Why Type? • Blood typing can show that two samples do not come from a certain person. This information is referred to as “exclusion” because it excludes a suspect. It is often used in forensics as well as paternity cases. • Blood typing can help to narrow down a list of suspects. • See packet for percentages of people with blood types.

  15. Probability of Blood Type • To determine the ratio of a specific blood type… • Determine the percentage of people with the type of blood (A, B, AB, or O). • A=41%, B=10%, AB=4%, and O=45% • If the Rh factor if negative (15%) of if it’s positive(85%). • Finally Determine whether the type is M,N, or S. • M=30%, N=22%, S= 48. • Multiply the probabilities of each factor. • For example: Type A, N, and negative Rh factor would be interpreted to mean that 1 out 74 people have this type (41/100 x 22/100 x 15/100 = 1/74).

  16. Testing Blood • Certain tests can determine whether a suspect is guilty. • The main component tested is hemoglobin. • Red, Carries oxygen • Iron-containing part called “Heme”, and a protein called “Globin”. • Hydrogen peroxide is used to show if blood is present. • Heme acts as a catalyst and breaks down the Hydrogen peroxide into Water and Oxygen gas.

  17. Blood Collection • Once the stain is verified as blood it is brought into a lab and analyzed. • Cut out portions stained with blood. • Place fingerprint tape over stain, rub non-sticky side with pencil, lift stain, and place tape on vinyl acetate. • Scrape bloodstain into an evelope. • Absorb the bloodstain with dampened cotton. • Presence of blood can also be tested for using Potassium salts, and acetic acid. • The blood will crystallize if the blood is in fact present.

  18. Bloodstain Patterns • Can Help to determine how a crime was committed. (where, what weapon used, manner in which it was used, and the extent of injuries sustained) • During or after a violent crime, blood may drip, spatter or squirt. Arced patterns may indicate a swinging motion from the perpetrator.

  19. Collection and Preservation of Bloodstain Evidence • First Photograph (the photos should show the position of the blood in relation to a permanent object as well as the relationship between different blood drops) • Pictures should be taken from normal prospective as well as from 90 degrees. • A scale should be placed next to the bloodstain to show actual size.

  20. Blood-drop Analysis • Shapely drops – the shape of a blood drop can indicate the distance from which the blood fell and the angle of its impact. When a drop is falling, it is primarily spherical in shape. (the smaller – the more spherical) • Drop acceleration – We can use this information to discover • The droplet’s speed at time of impact • Direction of the droplet’s travel • The approximate size of the blood drop

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