Philosophical Society of Washington

Minutes of the 2,254th Meeting

President Larry Millstein called the 2,254th meeting to order at 8:15 pm April 17, 2009 in the Powell Auditorium of the Cosmos Club.

Mr. Millstein introduced the speaker of the evening, Ms. Amanda C. Sozer of SNA, International. Ms. Sozer spoke on “Forensic DNA Profiling.”


Almost all cells have DNA. The only exceptions are mature red blood cells. It is the same in almost all cells. The only exceptions are sperm and egg cells, where only half the DNA is in each one. DNA provides the genetic instruction for the cell.

DNA is found in two places in the cell, in the nucleus and in mitochondria. These are organelles that provide energy for the cell. DNA profiles are really unique for all individuals except identical twins. The interest in forensics is in areas that are different for different individuals but the same for related individuals.

DNA occurs in two spiral strands along a phosphate “backbone.” There are four bases, adenine, thymine, guanine, and cytosine, often called A, T, G, and C. They occur in pairs across strands, A’s with T’s and G’s with C’s. Because of that, strands will bind with complementary strands, and they can find specific places in the strand by using probes.

You can’t see DNA, it is way too small, so they tag it with fluorescence. How they do that, she did not say.

Three kinds of DNA are used in identification, autosomal, y-chromosome, and mitochondrial. Autosomal comes from both parents, exactly half from each. If you have a bit of autosomal DNA that did not come from one parent, it came from the other. Y- chromosome DNA comes only from the father, and mitochondrial DNA is located in the egg and comes only from the mother. Both men and women get it all from their mother. It is more hardy than autosomal DNA because the pieces are very small. It is not as informative, because it is constant through matriarchal lineage, except for mutations, and has no relationship to partriarchal lineage.

Ms. Sozer distinguished DNA fingerprinting from DNA profiling. DNA fingerprinting is not used any more. It involved actually matching up strands of DNA. It was useful, but it required much more DNA than DNA profiling, which can be done with about 1/25th the quantity.

In DNA profiling, they use the fact that DNA sequences repeat certain numbers of times, and the number of times they repeat is determined by the DNA. People are characterized by the number of repeats.

They take different areas of DNA and make lots of copies of it. They heat the DNA to break it apart; then they add a DNA enzyme to get it to make another piece. They do this over and over, and each time the number of pieces is doubled. They give the DNA characteristics numerical values, and they can then compare profiles of known sources to samples of unknown origin. This works really well if they have an idea who they are looking for, such as in paternity testing or a crime with a suspect.

They do extensive documentation when they collect samples. They keep records of who collected, when, and how. They maintain carefully the chains of evidence.

Separating the DNA from other tissue is a lot like washing clothes. They use detergent, she said, much like Tide detergent. It may seem exotic, but it boils down to getting rid of the goop from around the DNA. Finally, they put it through a capillary machine to look at it with laser light.

She recalled a case where a murderer put a human body in a trunk. To confuse investigators, the murderer killed a dog and put it also in the trunk, and the DNA of interest was mixed up with the dog blood DNA. Now they use a machine that does a preliminary analysis to separate human DNA from other DNA.

They also use the property that DNA particles have a net negative charge. They apply a positive charge to attract them. Smaller particles arrive first, since smaller things can travel through the mix faster. She used an analogy of crossing a room at a party. If you have only a drink in your hand, you can move fast. If you were carrying a telephone pole, it would take a lot longer. Why you might walk through the party with a telephone pole, she did not say.

By convention, they look at certain areas of the DNA. Unfortunately, different countries use different areas. Patterns can be different in different geographical locations, just as they are for different races and lines of people.

They don’t do the statistical calculations of probability any more. They just use the convention that if the profile matches on 13 characteristics, they call that identity.

DNA is not foolproof. Quality is very important. The FBI has developed standards for processing samples. Training is important, and now there are competency and proficiency tests for processors. Interpreterrs need to know the limitations of DNA testing, how standards were developed and what that means, and how samples were collected. Today samples are collected using collected blood, or, even more often, a simple swab of the inside of the cheek. At crime scenes, a variety of methods is used.

Some people are DNA shedders, they give off a lot of it. It can be a problem having one of them in a lab.

DNA profiling is expensive, but it is cost-effective. They have learned a lot from it. For example, people arrested in Florida have often committed crimes in Virginia. Interstate 95, it seems, is a crime artery.

She discussed her experience with the aftermath of Hurricane Katrina. The problem was to identify a large number of bodies, and the challenge was increased by the state of decay of many of them. The job was given to the largest DNA lab in the country, but it was not ready for the scale. There were 200 unidentified bodies, and after seven months, they had 153 identifications. Much was learned about processing. They developed standardized methods of collecting, storing, and keeping records and physical evidence.

Forensic analysts don’t usually talk to families, but in this case they brought in genetic counselors to get the families’ confidence so they could get reference samples. One of the things they learned was that people are often not related the in way they think they are. That was in about 6% of the cases.

In the question period, she was asked, “How unique are DNA profiles?” It depends, she said. In the case of a father and son, they are practically identical with respect to the Y-chromosome, while they are completely different with respect to mitochondrial DNA, unless by chance they share a matrilineal ancestor.

When chromosomes split, do they split the same way? Not, they split randomly. Two siblings will share approximately 50% of the autosomal DNA.

Could DNA patterns substitute for social security numbers to identify people? It would be very expensive, she said. Libya is considering it. Maryland has a child identification program going. She mentioned again the importance of standardized methods in these efforts.


After the talk, Mr. Millstein presented a plaque commemorating the occasion. He announced the next meeting and the Joseph Henry Lecture. He encouraged guests to consider membership and encouraged support of the society in various ways. Finally, at 9:31 pm, he adjourned the 2,254th meeting to the social hour.

Attendance: 60
The weather: Slightly cloudy
The temperature: 14°C

Respectfully submitted,

Ronald O. Hietala,
Recording secretary


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