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Roberts v. United States

February 15, 2007; as amended March 1, 2007


Appeal from the Superior Court of the District of Columbia (F-771-01) (Hon. Robert I. Richter, Trial Judge).

The opinion of the court was delivered by: Farrell, Associate Judge

Argued November 28, 2006

Before FARRELL, RUIZ, and BLACKBURNE-RIGSBY, Associate Judges.

A jury found appellant guilty of first-degree sexual abuse, crediting the testimony of thirteen-year-old K.W. that in January 2001 he drove her to the Stadium Armory parking lot in Southeast Washington and forced her to have sexual intercourse with him. (Though unrelated to her, appellant lived in the same home as K.W.). Some weeks later, K.W. told her mother of the assault after the mother saw open sores on the child's genitals that were later determined to be herpes. Medical testimony allowed the jury to infer that the onset of the herpes coincided with when K.W. said she had been assaulted by appellant, who had the same form of herpes the child developed.

The issues in this appeal, however, arise from evidence of a DNA match which the FBI found after examining semen removed from panties K.W. had worn on the day of the assault and comparing it to the known DNA profile of appellant. Dr. Frank Samuel Baechtel, a DNA expert from the FBI forensic laboratory, testified that appellant's DNA profile and K.W.'s profile were both consistent with DNA found in the recovered semen. He further opined, using the FBI's highly conservative estimate for cases involving so-called mixed samples, that the statistical chance of finding a person at random who could have been a contributor to the DNA on the panties was no greater than 1 in 410,000 among four major population groups in the United States.

On appeal, appellant makes a variety of challenges to the admissibility of Dr. Baechtel's opinion, arguing particularly (a) that the expert could not properly offer a match-probability statistic without also providing the jury with an "error rate," that is, a "false positive probability to represent the chance of a false match caused by laboratory error" (Br. for Appellant at 21); and (b) that the FBI's method of interpreting mixed samples, which contain DNA of two or more persons, "relies on unwarranted assumptions about the number of contributors and their individual DNA profiles" (Reply Br. for Appellant at 7). We hold that, while appellant could properly explore these matters on cross-examination of Dr. Baechtel or through expert testimony of his own, in order to attack the weight of the DNA evidence, they did not render inadmissible Dr. Baechtel's opinion either that appellant could not be excluded as a contributor to the semen found on K.W.'s clothing or as to the probability of a random match.

Appellant further argues that the judge erroneously barred him from arguing to the jury an exculpatory interpretation of the DNA evidence that could reasonably be inferred from Dr. Baechtel's own testimony. Although we agree that FBI protocols furnished a basis for appellant to argue that genetic material found at a locus of one of the tested samples was exculpatory, any prejudice he suffered from the preclusion of that argument is insufficient to justify reversal. Finally, appellant contends that the admission of out-of-court statements of FBI forensic scientists through Dr. Baechtel's testimony violated his constitutional right to confront witnesses. Under a recent decision of ours, we agree that appellant's Sixth Amendment right was violated; but because he did not object to this evidence on confrontation grounds, we apply plain error analysis and again find no basis for reversal.


To put appellant's arguments concerning the DNA evidence in context, some background regarding how the FBI derived and interpreted the DNA samples and made its match estimate is necessary.

Methods of Analysis and Statistical Methods

The FBI laboratory used two complementary scientific techniques to identify the DNA found in this case.*fn2 The first technique, called polymerase chain reaction (PCR), takes a targeted segment of a DNA sample and duplicates it over and over again to create enough DNA material for analysis. The second technique involves determining the specific genetic variations, or "alleles," in the DNA samples at specific sites ("loci") along the DNA thread. The particular variations examined in this case are called short tandem repeats, or "STRs." They were examined at thirteen loci which have been adopted as a national standard for use in the Combined DNA Identification System (CODIS) established by Congress in 1994. The PCR-based analysis using the thirteen STR loci has been explained by the Supreme Court of New Hampshire as follows:

At each locus, an individual's genetic code contains a combination of chemical markers organized into a pattern. These chemical patterns repeat themselves and these repeats can be chemically cut apart from one another. At any particular chromosomal locus, an individual will have a characteristic inherited from each of his or her parents, known as an allele. Further, at any given locus, a person will have DNA with a specific number of repeats of these alleles from each parent. Thus, for example, a person's PCR-based STR DNA profile for a particular DNA locus could contain a ten-repeat allele from his or her mother and a twelve-repeat allele from his or her father. STR testing involves the examination of short repeats and distinguishes between individuals by comparing the number of repeats at certain loci.

State v. Whittey, 821 A.2d 1086, 1093 (N.H. 2003).

The FBI conducted the PCR/STR analysis using two commercial kits called Profiler Plus (Profiler) and Cofiler that together can identify all thirteen CODIS loci. Profiler and Cofiler are processed by an automated system, and the results of the process appear in the form of an electropherogram, or graph that displays a series of different-colored peaks of different heights.*fn3 A DNA analyst, or examiner, interprets the data displayed on the electropherogram to determine the DNA profile, i.e., the alleles seen at all the examined loci. Once the DNA profiles from the evidence and known samples are determined, the DNA profile from the evidence is compared to the DNA profiles from known individuals to see if any of those individuals can be excluded as possible contributors to the evidence (or "questioned") DNA. A person can be excluded as a possible source if he has a DNA allele at a locus that is not found among the alleles at the same locus in the evidence sample. But if the alleles in the known sample are consistent with the alleles in the evidence, there is a match, in the sense that "the donor of the known sample cannot be conclusively eliminated as the source of the questioned sample." Id. at 1094. The analyst then calculates how frequently the allelic profile found in the evidence would be expected to be seen in a defined population, as a benchmark for the significance of that match. "The profile frequency is simply the probability that an unrelated person chosen at random from the population would have the same DNA profile as the unknown sample." United States v. Trala, 162 F. Supp. 2d 336, 343 (D. Del. 2001), aff'd, 386 F.3d 536 (3d Cir. 2004), vacated on other grounds, 126 S.Ct. 1078 (2006).

Using databases the FBI has generated to approximate the actual frequencies of the alleles in various population groups, the examiner ordinarily calculates the probability of a random match by multiplying the frequency of each of the alleles in the profile (while correcting for limitations in current genetic knowledge about those frequencies).*fn4 However, for a mixed DNA profile, in which the number of alleles at a locus indicates the presence of two or more contributors and there is no way to distinguish among the contributors, the FBI essentially adds the frequencies of all possible combinations of alleles observed at the locus to obtain a combined frequency for that locus. Then the combined frequencies of the alleles at all examined loci are multiplied together to obtain the match statistic for the entire DNA profile. ...

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