Fishback v. People
| Decision Date | 26 April 1993 |
| Docket Number | No. 92SC68,92SC68 |
| Citation | 851 P.2d 884 |
| Parties | Jeffrey FISHBACK, Petitioner, v. The PEOPLE of the State of Colorado, Respondent. |
| Court | Colorado Supreme Court |
Richard F. Thurston, Solomon L. Leftin, Denver, for petitioner.
Gale A. Norton, Atty. Gen., Raymond T. Slaughter, Chief Deputy Atty. Gen., Timothy M. Tymkovich, Sol. Gen., John Daniel Dailey, Deputy Atty. Gen., Robert Mark Russel, First Asst. Atty. Gen., Deborah Isenberg Pratt, Asst. Atty. Gen., Denver, for respondent.
We granted certiorari to review the decision of the Colorado Court of Appeals in People v. Fishback, 829 P.2d 489 (Colo.App.1991), affirming the trial court's admission of identification testimony based on a comparison of deoxyribonucleic acid (DNA) obtained from the defendant's blood with the DNA from a semen sample recovered from the victim. The admissibility of DNA identification evidence is a question of first impression for this court.
The defendant was convicted of first degree sexual assault, second degree burglary, and mandatory sentence violent crime.
The evidence connecting the defendant to these crimes included the victim's identification of defendant, fingerprint evidence, and expert testimony that a DNA profile from seminal fluid obtained by a medical examination of the victim after the assault matched a DNA profile from a blood sample taken from defendant.
The trial court conducted an evidentiary hearing on defendant's motion to suppress the DNA typing evidence. At the hearing two witnesses testified: Dr. William Setzer, the director of the University of Colorado Health Sciences Center DNA Diagnostic Laboratory 1 who was qualified as an expert in the area of molecular biology, genetics, and "DNA testing"; and Dr. Lisa Forman, an employee of Cellmark Diagnostics, 2 who was qualified as an expert in population genetics. At the conclusion of the hearing, the trial court ruled that DNA typing evidence was admissible under both CRE 702 and the test articulated in Frye v. United States, 293 F. 1013 (D.C.Cir.1923).
The court of appeals affirmed, holding DNA typing evidence to be generally accepted within the relevant scientific communities and thus, admissible under the standard set forth in Frye. We affirm.
A basic understanding of the scientific principles and techniques underlying DNA typing is essential in order to understand the legal issues relating to its admissibility. DNA typing for forensic purposes utilizes a technique in which the characteristics of a suspect's genetic structure are profiled and compared to the genetic structure found in material such as blood, hair, or semen recovered from a crime scene. The two profiles are then compared to see if they match. If the two profiles match, the statistical significance of such a match is calculated to determine the likelihood of a match occurring between the profile derived from the crime scene sample and a third person who is not the suspect. The process by which this is accomplished can be divided into three parts: (A) The theory underlying DNA typing; (B) the techniques which apply that theory; and (C) the method of calculating the statistical significance of a declared match.
DNA is the material that determines the genetic characteristics of all living things. The significant feature of DNA for forensic purposes is that, with the exception of identical twins, 3 no two individuals have identical DNA. Furthermore, because DNA does not vary within a particular individual, a DNA molecule found in one cell will be identical to the DNA found in every other cell of that person.
In human beings, every cell that has a nucleus contains DNA which is distributed across forty-six sections of the nucleus of the cell. These sections are referred to as chromosomes, and they form twenty-three pairs: half of each pair are inherited from the mother, the other half from the father. These twenty-three chromosomes contain thousands of genes which comprise the total genetic makeup of an individual. "Alleles" are polymorphisms of a given gene, i.e., they vary from one individual to the next, and since each gene is represented by two copies (one from each parent) two alleles are inherited for each gene. When alleles that constitute a pair (or "genotype") differ, the person is said to be "heterozygous" for that allele. When a person inherits the same allele from both parents, that person is said to be "homozygous" for that allele.
A DNA molecule is a double helix, resembling a ladder that has been twisted which, if unraveled, would be approximately six feet in length. The "sides" of the ladder are composed of a chain of deoxyribose sugars and phosphates, while the "rungs" are composed of one pair of the following nucleotide bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). According to the "base pair rule," A can only bond with T and G can only bond with C. Thus, the order of the bases on one side of the rung will determine the order on the other side.
Each DNA molecule contains approximately 3 billion base pairs, or rungs, the vast majority of which (99%) do not differ from one human being to the next. It is this similarity in rungs which accounts for the human characteristics of human beings. Certain sections of the DNA molecule differ (i.e., they are allelic) from individual to individual, race to race, and ethnic group to ethnic group. These areas of variation are called "polymorphic sites." At some polymorphic sites short sequences of base pairs repeat in tandem, over and over again. The core sequence comprising a given allele is called a Variable Number Tandem Repeat (VNTR) and may contain just a few or as many as several dozen nucleotide bases. Because the number of times the core sequence of base pairs repeats may vary among individuals, the length of a given allele, measured in numbers of base pairs, may also vary. For instance, one person may have a particular allele in which a given core sequence repeats only ten times, whereas that same allele in another person may contain the same VNTR that repeats 100 times.
There are approximately three million alleles on each human DNA ladder. While all of these alleles are polymorphic, some are much more polymorphic than others. Forensic DNA typing utilizes a small number of highly polymorphic or "hypervariable" sites.
A DNA profile arrived at through the isolation and comparison of the lengths of several highly polymorphic alleles is known as restriction fragment length polymorphism (RFLP) analysis. 4 A DNA profile constructed by means of RFLP analysis is accomplished through the following techniques.
1. Extraction of DNA. The biological material that contains DNA must ordinarily be separated from the material in which it is found. Once separated, the DNA is extracted from the samples by a chemical treatment which releases the DNA. An enzyme is then added to digest cellular material other than DNA, rendering a purer DNA sample. 5
2. Restriction or Digestion. The DNA is then mixed with restriction enzymes which "cut" the DNA molecules into fragments at specific base sequences. These enzymes recognize particular sequences of base pairs and sever the DNA molecule at all sites along the three billion base pair length of the molecule where the targeted base pair sequence occurs. This results in numerous DNA fragments which can vary in length from a few base pairs to several thousand. 6
3. Gel Electrophoresis. Next, the DNA fragments are sorted by length through a process known as "agarose gel electrophoresis." The solutions of DNA fragments from the various sources are placed in an electrically polarized gel near the negative electrode. Because DNA is negatively charged, the fragments will migrate towards the positive end of the gel. They will do so, however, to varying degrees based on the length of the fragment: the shorter fragments, being lighter and less bulky, will travel faster and farther in the gel. Several samples are run on the same gel but in different tracks or lanes which run parallel to one another. In addition to the sample fragments, other fragments of known length are placed in separate lanes of the gel in order to facilitate measurement of the sample fragments. At the completion of electrophoresis, the DNA fragments are arrayed across the gel according to length. 7
4. Southern Transfer and Denaturing. Due to the difficulty of working with agarose gel, the fragments are transferred to a more functional surface through the "Southern Transfer" method. A nylon membrane is placed over the gel and, through capillary action, the DNA fragments permanently attach themselves to the membrane while occupying the same position relative to one another as they had in the gel. At the same time, the fragments are treated with a chemical which splits each base from its complement by "sawing" through the middle of each rung so that the base pairs are separated into two strands. 8
5. Hybridization. A technique is then employed in order to locate the highly polymorphic alleles contained in the fragments which are useful for forensic DNA typing. This is done by dipping the nylon membrane in a solution of various "genetic probes" which are single-stranded DNA fragments of known length and sequence designed to complement the single-stranded base sequence of polymorphic fragments from the defendant and the crime scene samples. The probes hybridize only to those DNA fragments which contain base pair sequences that are complementary to the base sequence of the probe. Usually three to five different probes are used to isolate multiple alleles. The genetic probes are "tagged" with a radioactive marker so that, after linkage with the half of the core sequence that was split in two, the position of those alleles...
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