Kathleen Z. Rooney
Blood evidence can be used to identify an unknown victim, comparing the blood type of the body to the recorded blood type of possible missing persons. If blood spatters or stains are left at the scene by a perpetrator, blood typing can help to narrow a field of suspects. In order to understand what a blood type is, students will need an overview of the type of cells found in blood and an introduction to the immune system.
Blood is composed of liquids and solids that are pumped by the heart through the body''s network of veins and arteries. The liquid part of blood is called plasma, and the solids are made of blood cells: red cells, white cells, and platelets. Platelets are cell-–like structures, formed in bone marrow, that help repair damage to blood vessels by creating clots to seal holes and allow the body to heal. Platelets and red blood cells have no nucleus and no DNA. White blood cells are active in fighting infections, attacking foreign cells and eating dead cells and bacteria. One of the ways that these white blood cells fight infections is to create proteins called antibodies. These antibodies have
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special shaped receptors that will attach to a foreign object entering the blood. This object -– whether cell, chemical, bacteria or virus, has a specific epitope (think ""key"") that fits a specific antibody (think ""lock""). Once the antibody attaches to the antigen it may neutralize the object or identify the object as trouble, for other cells to destroy.
Red blood cells have antigens on their surface. It is these antigens that determine blood types. There are combinations of antigens that create even more complexity in the distribution of blood groups. The basic groups are called A, B, AB and O. They are known as (+) or (-–) depending on the presence or absence of the antigen called Rh factor.
Each type has an attached probability, making a rather straightforward probability distribution, with slight variations across racial categories.
Introducing the lesson, students can play a blood typing game at the Red Cross website
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or Nobel Prize website
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. Both of these interactive games allow students to see the medical application of blood typing, where correct matching of antigen types is necessary for healthy outcomes. Once students see why and how blood is typed, we can apply it to forensics by observing the probability distribution of specific blood types.
Figure BLOOD TYPE BY RACE
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There are so many ways that this can be used in the statistics curriculum, that it is a great topic to explore early and often. Shown above is a great two-–way table that provides insight when examined in two directions. This table can be used in a discussion of independence. This distribution can also be revisited later in the course as we meet the chi-–square distributions.
Blood typing can only narrow probabilities to 1%, in the case of a very rare AB-– blood. That is not a small enough probability to base a criminal case on, although it often is used as crucial evidence. The unambiguous nature of physical evidence is very powerful. Unlike witness testimony, it has the backing of scientific fact, even if that fact is a probability. One example that students can examine is the Kenneth Waters trial for the 1980 murder of Katerina Brow. Blood evidence, along with testimony of his former girlfriends led to his conviction. Eighteen years later, DNA testing of the same blood evidence did not match Mr. Waters. The hard work of his sister Betty Ann Waters to free her brother, and his wrongful incarceration are the subject of the film Conviction.
Students will watch and discuss the film. Prior to screening, we will recreate the trial of Mr. Waters. Students will work in groups as the defense and as the prosecution. The evidence available at the trial will be available to use as students create their arguments. A third group will play judge and jury. Students should present the evidence and the jury should come to a decision. After the trial we will look at the Innocence Project page about Kenny Waters and view the film.
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DNA
Why is DNA a more powerful measure of identity than blood type? Blood matches a single protein attribute of the blood sample giving us a single probability of a match, while DNA uses multiple tags. A good analogy is cards. If I have a 2 of Hearts and I am trying to make a ""pair"" in a card game, I need to find either a 2 of Clubs, Spades or Diamonds. I have 3 chances in the 51 cards left, that gives me about a 6% chance. Now, if I want to get four of a kind, I have to multiply all of the probabilities as I pick the cards up. The probability of 4 of a kind is (3/51)x(2/50)x(1/49) = 0.005 %. The probability of making 3 matches drops to a thousandth of the probability of making one match.
DNA fingerprinting is a complex process that can yield extremely strong physical evidence. The probability of getting a match drops dramatically by looking at multiple distinctive areas within the DNA code of a sample. These areas within the DNA are called loci. These loci have been established by genetic researchers and with the United States Federal Bureau of Investigation (FBI) into a system called the Combined DNA Index System (CODIS). Depending on the lab and the specific requirements of the test, two sets of DNA will be compared for matches on 7-–13 loci. A match on all 13 strands is a very powerful match indeed.
DNA is a tremendously long chain of bonded pairs (base pairs) of nucleic acids. These base pairs have distinct combinations and patterns that create a code. Areas of the chain are partitioned into genes which are specific portions of the chain that can produce a particular protein. The idea of ""mapping the genome"" is the concept that people can attach specific meaning to the codes of portions of this chain. Each person has their own expression of that gene, called their genotype. By understanding the purpose of a piece of the chain, we can notice the similarities and differences in genotypes that create variation in protein production, and in turn variation in anatomy.
About 2% of the total sets of base pairs have been sequenced to attach meaning about their genes. The rest are sometimes referred to as ""junk DNA."" Within the junk, there are curious areas that have stutters of combinations of base pairs. In other words, the combinations go into a sequence of repeating themselves. These areas are called short tandem repeat loci or STR.
Between individuals, each STR can exhibit a variable number of repeats of DNA code. An allele is a specific length of repeats that the locus can take on. There are a small number of alleles for each locus. The genotype of each person at the locus is a pair of two alleles, one from the mother and one from the father.
Large samples of DNA have allowed probability distributions of the alleles to be determined and within these distributions, each allele has a specific likelihood to occur. In order to calculate the likelihood of a particular genotype, or combination of alleles, the probability of each allele is multiplied together. Lets say that a person has a genotype of 8,10, the 8 could be from the mother and 10 from father, or reversed. If the alleles are different, we multiply the individual probabilities together and then this number is multiplied by two because of the two ways in which it could occur. If the genotype is 12,12 with the same allele came from both parents, the probability is the square of the frequency of the allele.
The uniqueness of a person''s DNA follows from the extremely low probability that more than one person could have the same combinations along all loci compared. This is called a match probability. This probability is calculated using the product rule, and making the assumption that the loci are independent. Looking at one loci D8S1179, a sample of size 200 from the FBI was used to calculate these relative frequencies. The highest probability within this loci is for the individual to have 14 repeats, with a frequency of 0.3333. Taken together, the chance that the individual would display 14,14 at D8S1179 is (0.3333)
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=0.1111. About the highest probability at this locus would be a 15% chance with genotype 13,14 or 2(.2222)(.3333)= 15%
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Some particular genotypes would have a lower probability, some perhaps higher. If we use this 15% probability as an example for all 13 loci, the match probability for this sample would be 1/.15
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because we would multiply the 15% chance of any particular genotype, with the same probability at 13 locations. This equates to about one in 1.5 trillion chance. Given the world population is only 9 billion it is extremely unlikely that more than one human would have this same combination of genotypes.
Students will be introduced to the science of the DNA testing, and its use in the courtroom through PBS video and online teaching resources.
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Once DNA samples have been collected from the scene of a crime, the DNA sample must be copied and amplified to make it measurable. The process has several steps. First, the DNA is separated from the cell structure by using heat or chemicals. Next, specific enzymes are added to the DNA to cut the sample into fragments. The fragments with the loci that we wish to examine are amplified by making copies. This process is known as a Polymerase Chain Reaction or PCR. An enzyme that works with the fragment that we want copied is added to the DNA, and in a series of reactions, the number of copies is increased exponentially.
The DNA material is poured onto a laboratory gel, and electrical current is used to attract the chain of DNA through the gel. Depending on the weight of the specific allele, less repeats will be lighter, the fragments will travel more or less through the gel. The result is a length of DNA specific to the allele of the individual at the given loci. A set of these lengths is the DNA fingerprint. Depending on the number of matches, a probability of two sets of DNA matching is calculated by multiplying the probability of each allelic match.
We will create our own probability distributions of the major loci based on sample populations.
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Students will have the opportunity to do a simulation to compare DNA strands "recovered from a crime scene" with the DNA strands of "subjects". Attention will be given to the exponential increase in probability as multiple markers match.