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Tree Fingerprints

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SCIENCE 911: Tree Fingerprints

How did DNA from a tree help catch a murderer? The case against the suspect seemed closed, but a detective's keen sense of observation and a geneticist's persistence provided the key piece of evidence in a crime that might never have gone to trial otherwise. Frontiers reenacts the real-life case of the first time plant DNA is used to help solve a murder.

Curriculum Links
Activity: Chromosomes and the Law
For Further Thought




cell structure


nucleic acids


legal issues,


Step back in time for this scientific scenario. The year is 1956. A new law requires each presidential candidate to have a complete physical examination prior to the general election in November. As part of the exam, blood samples are taken and analyzed, with a check of chromosomal makeup. Note: At this time, the number of chromosomes in human mitotic cells is believed to be 48, not 46 as we now know.

Your Assignment:

Study the candidate's chromosomal makeup and make a recommendation to the electoral committee.
  • To complete this fictitious scenario, you will need a photograph of a mitotic spread, blank cards, scissors and glue. (See notes below.)

  • Arrange the chromosomes from the mitotic spread on the card.

  • How many chromosomes are on your card?

  • What was the expected number of chromosomes?

  • How are you going to organize each group of chromosomes?

  • Explain why you chose to organize the chromosomes as you did.

  • In this activity you view the world as if it is 1956, the year that J. H. Tjio and A. Levan published "The Chromosome Number of Man." The work of this research team demonstrated that the correct number of chromosomes per normal mitotic cell is 46, not 48.

  • The scenario presented here is fictitious. By law, there is and was no requirement like the one described in this activity. However, it illustrates some ethical considerations associated with science. A similar scenario could involve discovering a genetic marker for Huntington's disease, a syndrome that typically begins late in life. As a scientist, what would you do with such information?

  • Photographs of enlarged mitotic spreads can be purchased from scientific supply houses. For example, Carolina Biological Supply Co. sells "Human Biophoto Sheets" of normal and abnormal spreads. Students can work in small groups to complete the activity. If you prefer, you can conduct a discussion of the critical thinking questions without doing the activity.

  • In 1959, French biologist J. Lejeune published an article giving an account of the use of genetics as part of the definition of a syndrome. This discovery was a major factor in founding the field of cytogenetics, the branch of biology that deals with study of heredity and cellular components. Since then, this field has continued to grow rapidly. Scientists have developed numerous techniques for the preparation of chromosome spreads and for the identification of chromosomal anomalies. The most recent development has been the fascinating DNA fingerprinting technique used as evidence in forensic investigations.

CREDIT: Larry Gilbert, who prepared this activity, is beginning his 20th year of teaching at The Park School in Brooklandville, Maryland. Currently, he is teaching alternative forms of energy and human genetics in the middle school.

  • How are you going to explain in your report to the committee that your samples do not yield the expected 48 chromosomes in each mitotic spread?

  • Should you report findings that support the yet unpublished work of Tjio and Levan? Should you keep quiet until the work is published? Should you keep quiet until after the election?

  • Who should have access to genetic information? Individuals? Employers? The government? Insurance agencies?


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.