Opinion ID: 2262050
Heading Depth: 1
Heading Rank: 2

Heading: alleles, chromosomes, and polymorphisms the nature of dna identification evidence [4]

Text: The techniques of DNA typing are fruits of the revolution in molecular biology that is yielding an explosion of information about human genetics. NRC REPORT, supra, at 2. The opinion of the trial court contains a detailed exposition of the technology on which the prosecution relies in this case. Porter, 120 Daily Wash.L.Rptr. at 483-85; see also United States v. Jakobetz, 955 F.2d 786, 791-93 (2d Cir.1992), cert. denied, ___ U.S. ___, 113 S.Ct. 104, 121 L.Ed.2d 63 (1992). We attempt our own brief summary, in lay terms, of an esoteric but fascinating subject. DNA is sometimes called the blueprint of life. It contains the chemical instructions for all of life's processes, as well as the genetic code that defines who we are, what we look like, and where our talents lie. With the exception of identical twins, no two people have the same DNA. The makeup of one's DNA remains constant throughout one's life. In recent years, forensic technologies have been developed for removing the DNA from human cells discovered at crime scenes and for comparing the evidentiary sample with the suspect's DNA. We must determine whether these technologies pass muster under Frye. Embedded within the nucleus of virtually every cell of each human being's body are forty-six rod-shaped chromosomes. Half of these chromosomes are inherited from one's mother and half are inherited from one's father. Each chromosome has the shape of a twisted ladder or spiral staircase. The banisters of this staircase are made of phosphates and sugars, while the steps or rungs consist of base pairs, or pairs of amino acids bound together. A single DNA molecule  itself not a very large entity  contains about three billion base pairs. Located at specific sites, or loci, along each chromosome are large groups of base pairs known as alleles, or genes. Over 99% of these genes are identical among all human beings. These genes define us as persons, rather than animals, plants, or other forms of life. They account for the many shared characteristics of all human beings. The remaining genes  known as polymorphic genes because they vary in form from person to person  account for our unique characteristics as individuals. Many polymorphic genes are known to have definite functions: some are responsible for the color of our hair and of our eyes, some for the shape of our body and the type of our blood. Other polymorphic genes, however, appear to have no function whatever. These junk DNA segments, which typically consist of varying lengths of repeating sequences of base pairs, form the basis for the DNA identification evidence at issue in this case. The remarkable technology which has provided molecular biologists with an entree into the wonders of sub-microscopic exploration has not yet enabled them to compare every base pair in one DNA molecule with every base pair in another to determine conclusively that the two molecules are, in fact, identical. Forensic scientists, seeking to apply the new technology to identify the guilty and to vindicate the innocent, have developed a shortcut for making this determination. After extracting and cutting DNA from cells taken from the crime scene and from cells taken from the defendant, they isolate, from each sample, maternal and paternal fragments of junk DNA from four different loci, and measure the length of the repeating sequence in each fragment. In other words, rather than comparing every base pair in the two DNA molecules, they compare eight of the defendant's genes against the genes in the same locations on the evidentiary sample DNA to see if they are consistent with each other. This process is known as restriction fragment length polymorphism, or, more manageably, as RFLP. It is aimed at measuring the variable number of tandem repeats, a concept which DNA aficionados rattle off as VNTR. Because each person has thousands of polymorphic genes, it is theoretically possible for two people to have identical genes in these four locations on their DNA molecules but to have differences  perhaps even hundreds of differences  in other locations. Accordingly, once it has been determined that the defendant's DNA and the DNA from the evidentiary sample share identical patterns at all four of the locations examined, scientists calculate the possibility that the match is merely a coincidence and that the two samples did not actually come from the same person. Making that calculation is generally the role of human population geneticists. The particular system followed by the FBI and presented by the prosecution to the trial court is known as fixed bin analysis. Scientists utilizing this process first estimate, for each of the loci which was analyzed on the defendant's DNA, the fraction of people in the defendant's broad racial or ethnic group, e.g., Caucasian, Black American, or Hispanic, with genes at that locus identical to the defendant's genes. They then perform a series of multiplications to determine the fraction of the defendant's group with identical DNA fragments at all four of the loci examined. The resulting fraction is generally an astronomically low one. [5]