CHAPTER 18 FORENSIC MEDICINE AND MOLECULAR BIOLOGY

Genotype: the makeup of our genes, that is, our DNA

Phenotype: Characteristics due to the expression of our DNA. It is usually based on a complex interaction of several genes. It often refers to visible characteristics, but can refer to characteristics only visible with lab studies.

These are both used in forensic analysis (Fig. 18.3).

The fingerprint: These depend on more than one gene. Identical twins have identical DNA but do not have identical fingerprints. Various factors influence development, including development of fingerprints.

Identical twins: Two embryos develop by first splitting a fertilized egg into two cells. Thus, each has identical DNA. Fraternal twins arise from two separate fertilized eggs, and thus do not have identical DNA. Based on DNA or blood evidence you could not tell which identical twin committed a crime.

Retinal scans: This is a unique pattern of blood vessels on the back of the eyeball, the retina. It can be used as a security check. A computer can compare your scan to the data base.

Blood, sweat, tears, urine, semen, saliva: These contain cells which on their surface have proteins that can be analyzed. Antibodies to the specific proteins are used to analyze them.

Allele: A pair of genes sitting on the same site on a pair of chromosomes.

ABO and HLA are tests of phenotype, not genotype.

ABO blood group: Refer to proteins on red blood cells. Analysis of these can only exclude a person 15 - 20 percent of the time in criminal cases or paternity suits. See Figure 18-4.

HLA typing: HLA stands for human leukocyte associated antigens. These were first demonstrated on leukocytes (white blood cells). These surface proteins are expressed by a group of genes called MHC, major histocompatibility complex. They are expressed on the cells of all higher vertebrates. HLA matching is important in organ donation: graft rejection is mainly mediated by T cells against these "foreign" cell-surface proteins.

Blood type evidence:

ABO testing can exclude a person in 15-20 percent cases.

HLA testing can exclude a person in about 90 percent cases.

ABO and HLA combined can exclude a suspect in about 97 percent of cases.

We want to answer: Does the DNA at the scene match the suspect or victim?

To test for this we can use:

(1) DNA fingerprinting

(2) PCR and dot blot

(1) DNA fingerprinting:

Like fingerprints, DNA fragments show unique patterns from one individual to the next. No two individuals have identical twins with the exception of identical twins.

Restriction Fragment Length Polymorphism, RFLPs.

It is estimated there is one difference in every 1000 nucleotidase between unrelated individuals.

Because of these differences, particular restriction enzymes will cut the DNA sequences to different lengths in different individuals.

1. After using the restriction enzymes the DNA is run on a gel via electrophoresis (Fig. 18.7).

2. The DNA on the gel is transferred to nylon paper (Fig 18.8), and a radiolabeled DNA probe is used to bind to the fragments.

3. An autoradiogrph is made using radiation sensitive film (Fig. 18.9).

The DNA fingerprint made using this technique has five essential lanes: marker, victim, evidence, suspect, control (Fig. 18.10).

The markers are standardized DNA fragments and the control is DNA known to react reliably to DNA probes. If the control does not work correctly then you know the procedure has been messed up.

Fig 18.11 shows the autoradiograph from an actual criminal case. Lane 5, the blood stains from the defendant's jeans, and lanes 6 and 7, the blood on the defendant's shirt, matched V in lane 8, the victim's blood.

VNTRs: variable number tandem repeats.

These are short sequences of non-coding DNA which are repeated over and over, but different people have different numbers of repeats. Restriction enzymes are used to cut out the VNTR, and electrophoresis and Southern blotting are done, as before. The length of the fragment varies from person to person, the variability is enormous.

See Fig 18.12.

PCR is used when there is very little DNA or when the DNA is quite degraded.

Theoretically, PCR can be done from a single cell. The best fragments to amplify are small regions with high variability from person to person.

After the PCR is used to amplify the DNA strand, a dot blot test is used. In this test, the crime scene PCR-amplified fragment and suspect's corresponding fragment are bound to a membrane and tested with a DNA probe. The DNA probe is a complementary strand with a radioactive or fluorescent tag that is used to visualize the dots. If the probes bind to both, there is a match between suspect and crime scene.

If there is no match this is an exclusion. The evidence blood does not match the suspect or victim. The analysis is done and the DNA evidence says the suspect is innocent.

If there is a match, this is an inclusion.

In the case of inclusion, we now have to deal with probabilities.

What is the probability of this match occurring by chance?

To do this, calculate the probability of the genotype of the suspect as a random event. To do this, we look at loci, sites or locations on a chromosome that may be a gene, and RFLP or a VNTR.

Example of probability calculation, if looking at two loci:

PA1 X PA2 x PB1 x PB2

0.05 x 0.05 x 0.10 x 0.10 = 0.000025 or 1 in 20,000 (Also see page 293). There is a 1 in 20,000 chance that a person from the same population as the suspect could have this combination of four loci. The more loci, the more you can decrease the chance of the suspects allele combination occurring by chance in the population.

Of course, if their is a conspiracy or the jury strongly suspects a conspiracy, then the probabilities mean nothing.