It has long been an axiom of mine that the little things are infinitely the most important.
–Arthur Conan Doyle, A Case of Identity (Sherlock Holmes)
Despite the preceding statement, it is doubtful that Sherlock Holmes could have ever predicted that such a tiny little molecule inside our cells would become one of the most powerful tools in the forensic scientist’s arsenal. In recent years, however, DNA has become instrumental in the conviction of the guilty, the exoneration of the innocent, and the sorting out of historical mysteries. Beginning with the origins of serology testing in the 19th century through today’s automated DNA testing, certainly, it can be said that of all the significant advances made in the field of forensic science, forensic biology has made the greatest and most profound leaps.
Prior to the integration of DNA testing into the forensic laboratory, scientists who examined evidence for the presence of stains of biological origin were known as forensic serologists. The word serology refers to the science of studying serum, which is the clear yellow fluid found in separated whole blood. Forensic serology, however, also dealt with the analysis of semen, saliva, urine, and other physiological fluids. Therefore, the term forensic serologist really fell into disuse, as it did not fully explain the full gamut of work that these forensic scientists did. Today, the field has developed into a much broader one and is known as forensic biology. The job of the forensic biologist is to detect, identify, and type body tissues or stains left at a crime scene to ultimately determine to whom they belong.
The detection and identification process starts with a physical examination of the evidence collected at a crime scene. The examination begins with a careful visual examination of the physical evidence and is often coupled with alternate light sources to enhance the detection of any biological stains that might be present. Semen, for example, tends to fluoresce under certain types of light. This is what is being done when a forensic biologist or crime scene investigator uses what looks like a blue light to illuminate a piece of bedding or other item of interest. One important thing to remember is that during this process and all subsequent tests, it is imperative that detailed notes and documentation be kept, as mentioned in previous chapters.
After a stain has been identified, a presumptive test, which is a color-change test that screens for the presence of a biological fluid, is performed. These tests are quite sensitive, but not that specific; they do not confirm the presence of a particular fluid. (These presumptive color tests are similar in nature to those used in drug cases, as discussed in Chapter 9, “Drugs and Toxicology.”) Bloodstains, for example, often appear similar to rust or ketchup stains. To prevent unnecessary testing, a positive or negative presumptive test can steer the scientist in the right direction in a case to avoid unnecessary consumption of resources later on.
One such presumptive, or screening, test for the presence of blood is the use of Phenolphthalein, which is known as the Kastle-Meyer (KM) test. Upon the addition of hydrogen peroxide, the presence of hemoglobin (the oxygen-carrying molecule found in blood) causes, or catalyzes, an oxidative reaction to occur resulting in a color change from clear to pink.
It should be noted, however, that animal blood also results in a positive presumptive result and, therefore, this test does not give a definitive answer for the presence of human blood. Care must also be taken in interpreting presumptive test results for blood, as there is the possibility of false positives and false negatives. The only confirmatory tests for blood are microcrystal tests that produce unique crystals upon the addition of specific chemicals, which are known as the Takayama and Teichman tests. These confirmatory tests are rarely done in today’s forensic labs because once bloodstains are identified, they are usually sent for DNA testing. Since forensic DNA testing is only specific for primate DNA, the likelihood that DNA profiles from nonhuman blood will be used for evidence is miniscule. There is, however, the very remote possibility that gorilla saliva (from a primate) got mixed together with dog blood and produced a DNA profile. One must consider all possibilities in forensic science!
In addition to the KM test, there are several other presumptive tests for the presence of blood that forensic scientists use. Leucomalachite green is another example of such a color test; it changes from clear to green when blood is present.
Another presumptive test for blood that is notoriously seen on forensic television shows like CSI is the luminol test. Luminol is a type of chemical that gives off light in the presence of blood or, more specifically, exhibits chemiluminescence. Luminol is generally utilized at crime scenes by spraying it on surfaces such as walls, floors, or other structures where the presence of blood might be present. The visualization of luminol is best seen in the dark, so the lights need to be turned off and the windows need to be covered at a crime scene when performing this technique. Because of its high sensitivity, luminol is often used at crime scenes where it is suspected that the perpetrator has cleaned up. Even in cases where blood has thoroughly been cleaned off of walls, floors, etc., luminol can still pick up small traces of blood that were left behind, such as behind floor boards. Crime scene investigators can then pick up the blood trail, where it might otherwise have gone unnoticed. This can be particularly useful in homicide reconstructions and corroborating suspects’ statements. One drawback with luminol, however, is that it gives false positive results with oxidizing chemicals, like bleach. Also, with very dilute stains and the passage of time, even luminol can have its limitations. There are, however, new products, such as BLUE-STAR® FORENSIC latent blood reagent (www.bluestarforensic.com), which have proven to be even more effective in revealing latent blood.
One of the most probative pieces of evidence in a sexual assault case is the presence of semen. It is usually found in or on body orifices, underwear, condoms, or bedding. Depending on the type of substrate the semen is deposited on, locating it can be easy (as in the case of a dark surface, since it dries as a whitish stain) or more difficult (when it dries on light surfaces). An alternate light source is also often used to locate stains because of semen’s fluorescent properties under certain wavelengths of light.
Once a possible semen stain is located, a presumptive test is performed to determine if semen might be present. This is done by looking for the presence of an enzyme called acid phosphatase (AP), which is found in large quantities in semen. A positive presumptive AP test changes from clear to reddish-purple. The results, however, must be interpreted carefully, because the enzyme will lose activity over time and other bodily fluids contain AP, including vaginal secretions.
Once a positive presumptive test for semen is achieved, a confirmatory test is required. Semen is a mixture of fluid consisting of spermatozoa, or sperm cells, and seminal plasma. The visualization of sperm under the microscope, therefore, is one of the easiest ways to confirm the presence of semen. Another confirmatory test done in the forensic lab is the prostate-specific antigen (PSA), or p30, test. The forensic usefulness for PSA was demonstrated by George Sensabaugh of UC Berkeley in the 1970s. This test enables forensic scientists to demonstrate the presence of semen in the absence of sperm, such as in aspermic men or men who have undergone vasectomies. While PSA is found in low levels of other body tissues in both men and women, high levels of the protein are excellent indicators of the presence of semen. The one exception to this is that high levels of PSA can be found in men with prostate cancer.
One final word on rapists who think they can get away with a sexual assault by wearing a condom—they are wrong! Forensic scientists have tests for just about everything. There are now several laboratories that test for condom lubricants using infrared spectroscopy. (See Chapter 6, “Trace Evidence” and Chapter 9, “Drugs and Toxicology,” for a more detailed description of IR.)
Another type of bodily fluid that is screened for using presumptive testing is saliva. This type of physiological fluid is often present in sexual assault cases where biting or licking has occurred. Saliva screening utilizes a test to detect the presence of salivary amylase, an enzyme found in saliva that helps in the breakdown of saccharides, or carbohydrates, in the mouth while chewing. Again, care must be taken in interpreting these results because other body fluids can contain low concentrations of amylase, thereby giving false positive results. While saliva itself doesn’t contain cells, it does pass through the salivary ducts in the mouth and picks up oral epithelial cells containing DNA.
Once a biological fluid has been identified, the sample needs to be typed to determine its origin (see Figure 7-1).
Figure 7-1 Overview of DNA typing
Typing is the detection of a person’s genetic profile, which in the case of DNA is represented by a string of numbers. The difference in type between the evidence stain and an exemplar (a known reference sample from either the victim or the suspect) provides a positive elimination. If there is no elimination, the results must be evaluated against population frequency data to give a measure of the significance of the failure to exclude. In other words, how statistically relevant is it that two DNA profiles are included in a “match”?
Some of the features that make DNA such a good typing system are the following:
• It shows variability from person to person but is constant within one individual throughout their lifetime.
• It is stable in shed form.
• It can be detected at small concentrations found in forensic samples.
• Its frequency of occurrence within the population is known and stable.
DNA stands for deoxyribonucleic acid. It is the biological blueprint of life. The DNA molecule in its native form is double stranded and looks like a twisted ladder, or double helix. It is a continuous chain of repeating units made up of a sugar (deoxyribose) and a phosphate backbone attached to four chemical building blocks called nucleic acids or nucleotides. The sugar and phosphate group is what provides the handrail for the twisted ladder; the nucleic acids are what provide the rungs for the ladder. These rungs are made up of purines and pyrimidines. The purines consist of adenine (A) and guanine (G). The pyrimidines consist of cytosine (C) and thymine (T). Adenine always pairs with thymine, and guanine always pairs with cytosine (see Figure 7-2).
Figure 7-2 The molecular structure of the cross-linking nucleic acids
The human body is comprised of roughly three trillion cells. Inside most of these cells exists the blueprint, or instruction manual, in the form of DNA in the cell’s organization center, the nucleus. (Red blood cells, which carry the oxygen and make blood red, are so specialized that they do not require a nucleus or DNA.) It is this DNA that is responsible for such things as eye color, straightness of hair, and a host of other physical characteristics.
The DNA inside the cell’s nucleus is found in the form of long strands known as chromosomes. Human beings have 23 pairs of chromosomes, or 46 in all. Half of your chromosomes come from your mother and half come from your father. Along the chromosome are regions of DNA called genes, which act as units of heredity. Alternative forms of the same genes are called alleles. ABO blood types, for example, illustrate different alleles in people. People are type A, type B, type O, or, the most rare, type AB. The combination of alleles a person has determines their blood type. The location on the chromosome occupied by the allele is called a locus (or plural, loci). When two alleles at a locus are the same on both chromosomes, a person is homozygous for that trait. If they are different, that person is heterozygous. For example, a woman with type B blood can be either homozygous for the B allele (BB) or heterozygous (BO). Both allele combinations, however, still give her type B blood.
The first step in DNA typing after a biological stain has been identified is to isolate the DNA from the cells. This can be done in a number of different ways depending on the type of case involved, the amount of sample available, the type of specimen, and whether it is a biological stain, tissue, or bone. (Many people think that forensic DNA analysts work with powerful microscopes to “see” the DNA. This is a common misconception. The majority of work that is done is performed in small tubes with clear liquids where the forensic biologist is unable to “see” what is taking place.)
Because the cell contains many different components, the DNA needs to be separated from the other parts of the cell before it can be examined. This can be achieved in several different ways. Two of the most popular extraction methods are the Chelex® method and the organic method.
In the Chelex extraction method, heat and water are used to break apart the cell membranes and destroy cell proteins. Chelex, which is a suspension of tiny resin beads, protects the DNA from DNA-destroying enzymes, called nucleases, by rendering them inactive. The last step involves spinning the tube of DNA and Chelex in a very fast machine called a centrifuge. The centrifuge’s g-forces pull the heavier Chelex resin and cell debris to the bottom of the tube, while the separated DNA remains at the top of the tube in what is called the supernatant. This type of extraction works very well for forensic samples, such as bloodstains, buccal (saliva) swabs, and cigarette butts.
Organic extraction uses organic solvents, hence its name. In this method, chemicals are first added to break open the cell wall and to destroy proteins that protect the DNA. Next, phenol/chloroform and isoamyl alcohol are added to separate the proteins of the cells from the DNA. When spun at high speeds in a centrifuge, the unwanted proteins and cell debris are separated from the DNA into two layers. While this extraction method works well in recovering lots of DNA from difficult samples, it requires the use of hazardous chemicals, and is more time consuming than Chelex extraction.
The Chelex and organic extraction methods work quite well when only a single source of DNA is present. What happens, however, when a mixture of DNA from two individuals is submitted, as is often encountered in sexual assaults? How can an accurate DNA profile be generated?
Mixtures present a unique challenge to the forensic scientist. Differential extraction is one technique that is used to separate sperm cells from epithelial, or skin cells, as is often necessary in sexual assault cases. Once the male sperm fraction is separated from the female epithelial cell fraction, it is much easier to obtain the DNA profile from the rape suspect. This type of extraction exploits the morphology, or physical characteristics, of the sperm cells of the male to separate them from the epithelial cells, usually from the female (see Figure 7-3). (In the case of sodomy where the victim is male, the epithelial cells would be from a male.)
Figure 7-3 Differential extraction overview
The differential extraction of a mixture of cells uses different chemicals at different times. The procedure begins by first breaking open the epithelial cells and isolating the female DNA, followed by breaking open the sperm cells and isolating the male DNA.
As shown in Figure 7-3, the first step the forensic scientist performs is to cut the swab, or stained garment, and place the cutting into a tube (the swab is usually obtained from the sexual assault kit taken at the hospital from the rape victim). Chemicals are then added to the cutting and the tube is shaken vigorously to free the sperm cells and epithelial cells from the substrate. The substrate remains (i.e. the cotton swab or stain) is then removed from the original tube and placed into a clean tube. This becomes the substrate remains fraction. Since not all of the cells will come off of the swab, it’s expected that a mixture of female and male DNA will be found in this fraction. This mixed DNA profile from the sperm cells and epithelial cells can be used later if no clean male DNA profile can be obtained.
The next step in the differential extraction (rows 3 and 4 in Figure 7-3) involves adding chemicals to the mixture of sperm and epithelial cells that were removed from the swab or stain, to preferentially break open only the epithelial cells in the mixture. During this step, the sperm cells remain intact with their DNA enclosed, because the nuclear membranes of sperm are tougher than their epithelial counterparts. The female DNA can then be isolated as the epithelial cell fraction.
The final step in the differential extraction is to lyse, or break open, the remaining sperm cells, thereby freeing the male DNA contained in the sperm. This produces the sperm cell fraction.
These three separate fractions—substrate remains, epithelial cell, and sperm cell—then become subsamples of the original sample, and can be used by the forensic biologist to obtain or interpret the DNA profiles of the victim and the perpetrator. While most forensic laboratories use some form of a differential extraction procedure, there are several variations on the protocol depending on the lab.
Occasionally, the components of a mixture cannot be fully separated from one another in a differential extraction, and one is left with a mixture of DNA, as in the case of the substrate remains fraction. Forensic DNA analysts, however, can still deduce the major and minor contributors in a mixture case based on the ratios of DNA present. (A full explanation on the topic of mixture interpretation is beyond the scope of this book.)
After the DNA is extracted from the cell, the forensic scientist needs to determine how much DNA is present. This is important for the subsequent steps in the DNA typing process since they require precise amounts of DNA to be used. In quantifying the DNA, the analyst is also able to make certain that the DNA recovered is of primate origin, rather than from another source such as lower-order animals or bacteria. One common method of DNA quantitation in today’s forensic laboratories is known as the slot-blot technique, which uses the QuantiBlot® Human DNA Quantitation Kit by Applied Biosystems. In this procedure, the unknown sample is compared to known standard amounts of DNA to determine its quantity. Recently, however, many laboratories have been switching to a different quantifying technique called real-time PCR, which determines the quantity and quality of DNA present in a sample.
The next step in the DNA typing process is a procedure called the polymerase chain reaction (PCR). PCR is a method that enables a small amount of DNA to become amplified, or copied, into millions of additional copies. The technique, discovered by Nobel laureate and surfer Kary Mullis, essentially works like a Xerox machine; target DNA can be multiplied into millions of copies of the exact same DNA within a short period of time. The process involves repetitive cycles of heating and cooling of samples in a precise, temperature-controlled environment, which is regulated by a machine called a thermal cycler. Each cycle of PCR involves three distinct steps that are repeated over and over again until the desired amount of DNA is reached (see Figure 7-4).
Figure 7-4 PCR amplification overview
The first step in a PCR cycle is called denaturation. During denaturation, the DNA strands separate from one another at a very hot temperature of 94°C. The next step is called annealing, which cools the DNA to 60°C. During annealing, small fragments of DNA, called primers, bind to the DNA template and target the region of interest of the DNA to be amplified. The final step, extension, extends the primers on both strands by copying the target region of DNA with DNA building blocks, and occurs at 72°C.
At the end of the first cycle, two copies of the target DNA are generated. When this cycle is repeated, the number of targeted DNA molecules doubles. Usually, an entire PCR run is 25 to 35 cycles and takes 2 to 3 hours. If the average PCR is 28 cycles, then the yield of DNA will be millions of copies (228).
In the mid-1980s, Sir Alec Jeffreys discovered that every person’s DNA is unique. He and his colleagues found that certain areas of the long human DNA molecule are polymorphic, or exist in different forms between one person and the next. These variable areas on the DNA molecule allow forensic scientists to individualize people, or distinguish people from one another.
In the human genome, our DNA is full of repeated sequences. These sequences are encountered both in different lengths and different numbers of repeats. If one visualizes a train as being representative of our DNA, different trains can have different numbers of cars, and different sizes of cars. DNA regions with repeat units that are used in human identification are called short tandem repeats, or STRs. They are short because they average four base pairs in length. They are tandem because they occur next to each other. And they repeat because the same base pair unit is repeated multiple times in the DNA sequence. What is significant about all of this is that the STR markers chosen by the forensic community are highly variable between people and can be used to discriminate you from every other person in the world. This means that, just as the old adage says, you are unique, just like everybody else! The exception, of course, is identical twins, who have the same DNA. (As a result, there have been accused murderers who have pointed the finger at their long-lost twin brother.)
The forensic DNA community has developed a common naming system for these STRs to allow laboratories to share this DNA information with one other. This allows labs across the world to communicate in a common language with regard to DNA. There is currently a standard set of 13 core forensic STR loci, or locations, that are used across the United States. Systems in other countries, such as the system used in Great Britain, use some of these 13 loci, in addition to other loci. Today, there are commercial kits that labs can purchase that target and amplify these standard sets of core forensic STR loci. The 13 core forensic loci currently used in the United States are the following: D3S1358, D16S539, THO1, TPOX, CSF1PO, D7S820, vWA, FGA, D8S1179, D21S11, D18S51, D5S818, D13S317. Aside from these 13 locations, STR tests also screen for the gender-determining locus, amelogenin.
How does all of this work? After the PCR amplification process previously described, there is a mixture of DNA molecules (different STR markers) that require separation to make sense of all this information. In other words, in any given PCR reaction, there are many DNA fragments that need to be resolved from one another. The separation step that does this is called electrophoresis. Because DNA is a negatively charged molecule, it has an affinity for a positively charged electrode. (Remember, opposites attract.) When an electric field is applied to a DNA mixture, the DNA starts to migrate across the field from the negative end toward the positive end. The DNA mixture contained in each sample is added to a gel with a series of small pores that allows the DNA fragments to migrate through it. The smaller DNA fragments tend to migrate through the gel faster than the larger DNA fragments.
This process can be performed in two different types of electrophoresis environments: slab-gel electrophoresis and capillary electrophoresis. Both types of systems are used in forensic labs, with the trend moving more toward the capillary systems.
In the slab-gel electrophoresis system, wells are made in a solid gel for loading DNA samples. The gel is then placed in a buffer solution. Each sample is then loaded into a different well. Known and unknown samples are never loaded on the same gel, to prevent contamination and sample mix-up. Next, an electric current is applied to the gel and the DNA fragments begin to migrate. A very expensive instrument called a Genetic Analyzer, made by Applied Biosystems, then “reads” these separated fragments using a laser. During the PCR step, the DNA fragments are tagged with different-colored fluorescent dyes that allow the laser to differentiate between the different DNA fragments. Computer software finally takes this information and converts it into bands (similar to the supermarket barcodes) and peaks. These peaks are assigned numbers based on the number of repeat units in the DNA molecule. Taken together, these peaks and numbers make up an electropherogram, or DNA profile (see Figure 7-5).
Figure 7-5 Electropherogram of DNA profile
In the capillary electrophoresis system, instead of using a slab of gel, a very thin tube called a capillary is used to perform the electrophoresis, or DNA separation. A polymer, which is a gel-like compound with tiny passageways, is pumped into the capillary, allowing the mixture of DNA fragments to be separated based on size. A laser then also “reads” these DNA samples and software converts this data into electropherograms. The advantage of the capillary system over the slab-gel system is that the capillary system is faster and more samples can be run at the same time (16 samples can be run in 45 minutes).
In some cases additional, or specialized, DNA testing might be warranted. When there are multiple male suspects involved in a case, when an autosomal (non-sex determining) DNA profile could not be obtained, or when there is too little DNA present, other DNA tests can aid in the investigation.
Y STRs are found only in males, because males possess an X and a Y chromosome, whereas females have two X chromosomes only. Since the majority of crimes in which DNA evidence is used involve sexual assaults, and since the overwhelming majority of sexual assaults involve male perpetrators, DNA tests that can exclusively examine the male DNA portion are quite beneficial. Cases in which Y STR testing might be used are those that involve very high levels of female DNA mixed with small amounts of male DNA, or where the number of men involved in a “gang rape” is unknown.
While Y STR testing can prove quite valuable in certain types of cases, the results are not as meaningful as regular, or autosomal, STRs. This is due to the fact that the Y chromosome is transferred directly from father to son. A match between evidence and a suspect only means that the suspect could have contributed to the sample, as could have his father, brother, son, uncle, or even long-lost paternal cousin. The random match probability, therefore, is much lower for Y STRs and provides a lower power of discrimination between men.
There are also other popular uses for Y chromosome testing, including tracing human migration patterns, researching genealogies, and proving ancient genetic links, such as the link between modern day Cohanim to the ancient Jewish priesthood beginning with Aaron (Exodus 40:15).
Mitochondrial DNA (mtDNA) is another type of DNA that is useful in forensic investigations. This type of DNA is found in another part of the cell, called the mitochondria. The mitochondria is responsible for energy production within a cell. MtDNA is less susceptible to degradation than nuclear DNA, due to the fact that it is much smaller, is protected inside the double membrane of the mitochondria, and exists in a circular genome (see Figure 7-6), unlike the linear double-helical structure of nuclear DNA.
Figure 7-6 mtDNA genome
MtDNA molecules are also present in thousands of copies per cell, unlike its nuclear DNA counterpart, which has only two copies. Therefore, biological evidence that has been subjected to harsh environmental conditions may yield successful mtDNA typing in cases where nuclear DNA typing has failed.
Another feature of mtDNA is that it can be used to determine ancestry, because it is inherited maternally. This means that you have the same mtDNA type as your mother, who has the same mtDNA type as her mother, and so on. (Nuclear DNA is inherited in equal parts from both parents.) This is because mtDNA is inherited only from the egg cell of the mother. When a sperm cell fertilizes an egg, only the nuclear DNA contained in the sperm head, and not the mtDNA contained in the “neck” of the sperm, makes it into the egg. Thus all the cell components of the developing fertilized egg come from the mother, including the mitochondria. This trait is particularly useful is missing person cases, because a maternal relative can be used as a reference sample in place of the missing person. MtDNA is also extremely stable and undergoes very few mutations, which means that you can accurately trace your maternal lineage back many generations.
The mitochondrial genome contains about 16,500 bases and contains a noncoding hypervariable control region. Within this region, two segments, called hyper-variable region 1 (HV1) and hypervariable region 2 (HV2), tend to mutate at a higher frequency than the rest of the molecule. It is for this reason that this region is of particular significance for forensic individualization.
Just like Y STRs, however, mtDNA has a low power of discrimination. The probability of mtDNA being unique is usually only around 1 in several thousand depending on the size of the database being used to calculate the number of times that the mtDNA profile appears in the population. MtDNA is, however, an excellent way of excluding a person from being the source of a particular sample.
A new breakthrough in DNA testing that has been introduced into crime labs is the ability to obtain DNA profiles from very small amounts of sample. This type of testing is usually reserved for samples that contain only a handful of cells. Because of the small amount of DNA present in these types of samples, additional PCR cycles are run. To this end, some jurisdictions have begun analyzing property-related crimes like burglaries and robberies where only skin cells might be present. A burglar or robber that handled a stolen piece of property might have left partial or smudged fingerprints containing epithelial, or skin, cells from their hands or fingers. This evidence can then be submitted to the forensic lab for testing. A word of caution, however: low copy number (LCN) DNA results need to be interpreted more carefully since the sensitivity of the DNA typing is so much higher, and thus the chance of contribution from contamination is greater.
Statistics are an integral part of forensic DNA. Without reporting statistics, the results of a “match” would be insignificant. For example, if a scientist were to testify to the fact that the suspect has type A blood and the blood found at the crime scene was also type A, what conclusion is the jury supposed to draw from this? Hopefully, not much, because approximately 1 out of 4 people have type A blood.
DNA evidence, however, has much higher statistical weight. The statistical weight of a full 13-locus profile is generally greater than 1 in a trillion. This means that if one were to choose a person at random from society, the odds of that person having the same DNA profile in question would be greater than 1/1,000,000,000,000. Since the population of planet earth is approximately 6 billion (6,000,000,000), it would take around 166 planet earths to obtain a trillion people. To put it a different way, the odds of you winning the lottery are much greater than finding another person with your exact same DNA.
The reason that these odds are so small is that the probabilities of the individual 13 profile types are independent of one another, since the 13 core loci are located on different chromosomes. This means that the individual odds can be multiplied together to produce staggering odds. Statistics for mtDNA and Y STR DNA profiles require that a counting method be used instead of multiplying probabilities because the individual DNA markers are not independently occurring. The counting method produces smaller odds that a profile is unique.
The Federal Bureau of Investigation’s Combined DNA Index System (CODIS) uses forensic science and the power of computers as a tool to solve crimes. CODIS allows local, state, and federal crime labs to exchange DNA profiles electronically in hierarchical levels (see Figure 7-7).
Figure 7-7 Combined DNA Index System (CODIS)
CODIS enables different jurisdictions to link crimes to each other and to convicted offenders. DNA profiles begin at the local level, with local crime labs generating DNA profiles for the Local DNA Index System (LDIS). Local laboratories then upload their DNA profiles into the State DNA Index System (SDIS). Currently, all 50 states participate in the CODIS system. States then upload their profiles into the National DNA Index System (NDIS) maintained by the FBI. According to the FBI’s website (www.fbi.gov), as of April 2006, the NDIS database has 3,275,710 DNA profiles, including 136,672 forensic profiles and 3,139,038 convicted offender profiles.
CODIS allows scientists and investigators to compare a DNA profile with convicted offenders and other crime scenes. This is important because it can help link cases to one another across jurisdictions that may otherwise have gone unsolved. As the database grows in size, the ability for law enforcement to solve crimes will also increase. As of February 2006, CODIS has produced over 30,000 hits assisting in more than 31,700 investigations. In addition to the NDIS database, CODIS now also includes a missing persons database (CODISmp) and a mitochondrial DNA database (CODISmt).
Other countries throughout the world have also developed, or are developing, similar national databanks that operate like CODIS. Each country, however, has different statutes regarding who is required to submit a DNA profile, what is required for a profile to be expunged from the database, and which STR loci make up a DNA profile. To this end, INTERPOL (International Criminal Police Organization) has been working on the standardization and internationalization of DNA profiling worldwide. As a result, INTERPOL has been able to assist in international investigations involving national DNA databanks from multiple countries.
The DNA identification efforts for the victims of September 11, 2001 have become the largest forensic investigation in history. On that day, at the World Trade Center in New York City, 2749 people were murdered, leaving approximately 20,000 pieces of human remains. The goal was and still is to identify as many of those remains as possible in order to return them to their loved ones. Without the capabilities of DNA testing, however, this would largely have been an impossible task and the majority of victims would not have been able to be identified.
Analyzing samples from the World Trade Center site was only half of the process. The other part of the ID effort required the analysis of DNA samples provided by families. This included DNA of the victims from toothbrushes, razors, hairbrushes, and the like. In the absence of these, mouth swabs from biological relatives were used in a process called kinship analysis (see Figure 7-8).
Figure 7-8 Kinship analysis overview
By obtaining the DNA profiles of the mother and father of the deceased/missing or the DNA profiles of a spouse along with the victim’s son or daughter, identifications were made by applying rules of genetic inheritance and statistics.
Since a more detailed account of the World Trade Center identification efforts are beyond the scope of this chapter, you are encouraged to read Who They Were: Inside the World Trade Center DNA Story: The Unprecedented Effort to Identify the Missing (Free Press, 2005), written by the former director of the Forensic Biology Department for the New York City Medical Examiner’s Office, Dr. Robert Shaler.
Forensic DNA evidence can link a suspect to a crime, exculpate falsely accused suspects, recognize serial crimes and distinguish copycat crimes, identify the remains of victims, and reconstruct crime scenes. Parentage testing is a form of identity testing that has increasingly turned to DNA testing in recent years. People researching their genealogy have also turned to DNA typing to help establish a link to their past. History is also being rewritten based on the DNA testing surrounding historic figures, such as Jesse James, the unknown soldier at the Tomb of the Unknown, Czar Nicholas II, Thomas Jefferson, and many others. Forensic DNA testing has come quite a long way since its inception and will continue to make giant strides in the future.
Many of the details in forensic DNA typing are beyond the scope of this chapter. You are encouraged, however, to learn more about DNA from the website www.dna.gov, which was established by the President’s DNA Initiative, in conjunction with the National Forensic Science Technology Center.
Refer to the text in this chapter if necessary. Answers are located in the back of the book.
1. The source of the p30 enzyme found in semen is:
(a) The liver
(b) The testes
(c) The prostate
(d) The vas deferens
2. Spermatozoa can be visualized on microscope slides to prove the presence of:
(a) Blood
(b) Saliva
(c) Semen
(d) Feces
3. Purines are:
(a) Adenine and thymine
(b) Thymine and guanine
(c) Cytosine and guanine
(d) Guanine an adenine
4. Amylase is found in high concentrations in what stains?
(a) Semen
(b) Blood
(c) Urine
(d) Saliva
5. The rarest blood type is:
(a) A
(b) B
(c) AB
(d) O
6. Thymine is a:
(a) Purine
(b) Pyrimidine
(c) Polymer
(d) Phosphatase
7. The reaction of luminol with blood is best described as a:
(a) Antigen-antibody reaction
(b) Cold-fusion reaction
(c) Sympathetic reaction
(d) Chemiluminescent reaction
8. CODIS is an acronym for:
(a) Convicted Offender DNA Index System
(b) Combined DNA Identification System
(c) Convicted Offender DNA Identification System
(d) Combined DNA Index System
9. Pyrimidines are:
(a) Adenine and thymine
(b) Thymine and guanine
(c) Cytosine and guanine
(d) Thymine and cytosine
10. Which of the following is a positive presumptive test for blood?
(a) Luminol
(b) Leucomalachite green
(c) Phenolphthalein
(d) All of the above
