Category Archives: DNA

Unraveling DNA Mixtures

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DNA mixture

 

The New York Times’ Liz Robbins recently reported a development in forensic DNA analysis. Over the past several decades, improvements in synthesizing DNA from trace amounts greatly increased the sensitivity of DNA profiling. Yet increasing sensitivity does not provide a solution to cases in which evidence contains trace amounts of DNA from several people.

Theresa A. Caragine and Adele A. Mitchell of the New York City medical examiner office’s forensic biology lab may have a solution. Their Forensic Statistical Tool is an algorithm for a software program that enables analysis of a DNA mixture uncovered from a crime scene and determines the probability that the DNA brew includes a defendant’s DNA profile.

Before judges allow results of the technology in court, the technique must survive Frye hearings. A Frye hearing is a challenge to the general scientific acceptance of new technology. Several Frye hearings are scheduled in New York courts.

Bacterial DNA Profiling

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 Finding DNA

Earlier this month, researchers at Washington University School of Medicine (St. Louis) and at the European Molecular Biology Laboratory (Heidelberg, Germany) reported a new type of DNA profile: DNA profiles from human gut bacteria. The researchers analyzed microbial DNA in 252 stool samples from 207 individuals, focusing on 101 species of microbes commonly found in the intestine. They found many types of DNA differences, the sort of differences that generate unique DNA profiles. For 43 subjects, the researchers collected two stool samples one month to six months apart. The scientists found little variability in the microbial DNA. In short, they discovered variability in DNA profiles between subjects and consistency in DNA profiles in subjects over time.

“The microbial DNA in the intestine is remarkably stable, like a fingerprint,” said George Weinstock, associate director of The Genome Institute at Washington University. “Even after a year, we could still distinguish individuals by the genetic signature of their microbial DNA.”

It’s not immediately obvious how this discovery could be incorporated into a crime story. But, if you can do it, then consider yourself on the cutting edge.

A more apparent application of bacterial DNA profiling to mystery stories is found in the discoveries of Noah Fierer and his colleagues at the University of Colorado in Boulder. They found that a typical human hand shelters about 150 species of bacteria. The types of bacteria living on skin vary greatly. In a 2008 study, they identified more than 4,700 different bacteria species living on the hands of 51 people. Yet only five species lived on the skin of every participant of the study.

In 2010, the researchers reported that computer users leave DNA traces of bacteria on computer mice and keyboards. These DNA traces more closely match the DNA of bacterial colonies that inhabit the hands of the individual who used the computer, compared with bacterial DNA traces of randomly selected people. The researchers obtained useful samples of bacterial DNA two weeks after a person touched computer equipment.

Digging Mona Lisa

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Who posed for Leonardo da Vinci’s “Mona Lisa”? CNN’s Ben Wedeman reported that scientists may soon find the answer to this age-old question.

In Florence, Italy, old records indicate the burial site for Lisa Gherardini, the second wife of Florentine silk merchant Francesco del Giocondo. That’s Lisa as in Mona Lisa. The records led to a derelict building standing on the remains of a Franciscan convent.

Silvano Vinceti and his team exhumed and identified Lisa Gherardini’s remains. Bone fragments will be sent to universities, where researchers will analyze DNA and compare the DNA profile with DNA profiles of two confirmed relatives of Gherardini.

“Once we identify the remains,” Vinceti told CNN, “we can reconstruct the face, with a margin of error of 2 to 8 percent. By doing this, we will finally be able to answer the question the art historians can’t: Who was the model for Leonardo?”

While these efforts may reveal the true face of the Mona Lisa model, the mysterious smile is another matter. The smile did not belong to Gherardini, Vinceti claims. Rather, da Vinci lifted the smile from his longtime assistant, Gian Giacomo Caprotti.

Hunting Burglars with DNA Tests

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DNA

Source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber.

Forensic DNA analysis is usually cast as an important tool for the investigation of a violent crime. However, investigators also use the technology to solve property thefts.

The September-October 2012 issue of Evidence Technology Magazine features an article by Joseph Blozis on “Using DNA to Fight Property Crime.” He explains how New York City’s Biotracks program demonstrated this new use of DNA testing. The NYPD recovered DNA samples from burglary cases with no suspects and forwarded the samples to private companies for analysis. The Office of the Chief Medical Examiner Biological Laboratory then uploaded DNA profiles to the FBI’s CODIS system. Eventually, the efforts led to the identification of burglars in no-suspect cases and established links between apparently unrelated burglaries.

The successes of the NYPD program and similar programs in other jurisdictions have proven the value of DNA testing to solve property crimes. Finding the funds and staff to run extra DNA tests is another challenge.