Fly Forensics

1994 Woodrow Wilson Collection

Freddie's Friendly Fruit Market has recently been plagued by one rogue fruit fly that has proven resistant to Freddie's fly surveillance and destruction mechanism (Freddie's Fly Swatter). Contamination of fruit will have a negative impact on Freddie's business and he must catch the criminal fly before he has a chance to breed and produce more flies who may inherit the characteristics which make them threats to the fruit market. Modern forensic techniques can be used to help Freddie determine the guilty fly. There are four chief suspects in the fly caper. A description of the suspect follows on the next page. Freddie has found a bristle hair from the crime scene. DNA forensic evidence involves a technique called DNA fingerprinting, which identifies an individual by his or her unique DNA sequences. No two flies (or people for that matter) have the same DNA sequences, unless they are identical twins. Sources of DNA samples could be skin cells, blood, semen, or the roots of hair.

DNA Fingerprinting

To prove guilt in criminal cases, comparison between the DNA samples left at the scene of a crime with the suspect's DNA is accomplished with the help of a process called gel electrophoresis. First, the DNA from each sample is digested using chemical scissors, called restriction enzymes, which cut the DNA at specific sequences of nucleotide bases. After cutting, the DNA is now in fragments of different lengths. The process of gel electrophoresis separates the DNA fragments according to size and creates a pattern. This pattern is transferred to a nylon sheet and exposed to radioactively-labeled probes that will be attracted to areas of the DNA that will establish identity. After the radioactive probes have been added, the nylon sheet is placed against X-ray film and processed. The result is a series of black bands which appear where the probes had been attracted. This pattern of black bands produces a unique DNA fingerprint. Comparing DNA fingerprints from a crime scene with the DNA fingerprints of suspects will prove the identity of the criminal.

Crime Scene Evidence

DNA was extracted from the cells at the base of the bristles of each suspect fly and DNA was extracted from the cells at the base of the bristle taken from Freddie's Friendly Fruit Market. The DNA was cut with restriction enzymes (chemical scissors). Segments of the DNA were separated by a process called gel electrophoresis, transferred to nylon sheets, labeled with radioactive probes and exposed to X-ray film. The results of these DNA fingerprints can be seen on the next page. Analyze the results and answer the questions which follow.

Suspects And Evidence


Bugsy-a red-eyed, normal-winged male fly, whose mother had vestigial wings. He is homozygous for gray body, but is heterozygous for normal antennae. His father had Antennapedia. Antennapedia is an unusual phenotype in which antennae on the head have been replaced by fully developed legs. Bugsy is known for his circular flight patterns over decaying fruit. His favorite fruit is ripe peaches.

Dino-a white-eyed male fly with vestigial wings. He has the recessive trait of ebony-colored body, and is homozygous for normal antennae. He enjoys frequenting the kitchens of Italian restaurants, and ripe grapes are his favorite food. He is an excellent jumper but he has difficulty flying. Daisy-a red-eyed normal-winged female. Daisy's mother had vestigial wings and her father had white eyes. Daisy has an ebony body and has normal antennae, but her mother had Antennaepedia. Daisy enjoys visiting farmer's markets and loves ripe pears.

Tootsie-a white-eyed normal-winged female. Tootsie's mother had vestigial wings and an ebony body. Tootsie has a gray body and Antennapedia. Tootsie enjoys ripe plums and often stands on her head.

Fly Forensics

  1. From the evidence presented, who is the guilty fly? Explain how you reached your conclusion.

  2. Show the genotypes for each fly below. Use the symbols given to represent the characteristics.

    R= red eyes
    r=white eyes
    (Eye color is
    wings w=vestigial wings
    G=gray body
    g=ebony body
    A=normal antennae

    Fly Genotype

  3. What would be the expected genotypic and phenotypic results if the following fruit flies were crossed? Show all your work.

    A. A cross between Bugsy and Daisy
    B. A cross between Dino and Tootsie
    C. A cross between Tootsie and Bugsy
    D. A cross between Dino and Daisy

  4. Do you believe that a DNA fingerprint match is sufficient evidence when used in criminal cases? Explain your answer.

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