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Introduction
Several human genetic disorders are caused by extra, missing, or damaged chromosomes. In order to study these disorders, cells from a person are grown with a chemical that stops cell division at the metaphase stage. During metaphase, a chromosome exists as two chromatids attached at the centromere.
The cells are stained to reveal banding patterns and placed on glass slides. The chromosomes are observed under the microscope, where they are counted, checked for abnormalities, and photographed. The
photograph is then enlarged, and the images of the chromosomes are individually cut out. The chromosomes are identified and arranged in homologous pairs. The arrangement of homologous pairs is called a karyotype. In this investigation, you will use a sketch of chromosomes to make a karyotype. You will also examine the karyotype to determine the presence of any chromosomal abnormalities. The problem: How can chromosomes be observed?

Read the entire investigation. Then work with a partner to answer the following questions in lab.
1. What clues to the presence of certain genetic disorders can be seen in a karyotype?
2. Why might a laboratory worker attempting to diagnose a genetic disorder prefer to work with photographs of chromosomes rather than the chromosomes themselves?
3. Why would it be much more difficult to construct a karyotype of unstained chromosomes?
4. Which pair of chromosomes can contain two very different chromosomes and still be considered normal? Explain your answer.
5. How do autosomes differ from sex chromosomes?

Safety Precautions -- Be careful when handling sharp instruments.

Procedures
Part A. Analyzing a Karyotype

1. Observe the normal human karyotype in Figure 1. Notice that the two sex chromosomes, pair number 23, do not look alike. They are different because this karyotype is of a male, and a male has an X and a Y chromosome.

2. Identify the centromere in each pair of chromosomes. The centromere is the area where each chromosome narrows.

Part B. Using a Karyotype to Identify a Genetic Disorder

1. Study the human chromosomes in Figure 2. Notice that 23 chromosomes are numbered 1 through 23.
2. To match the homologous chromosomes, look carefully at the unnumbered chromosomes. Note their overall size, the position of the centromere, and the pattern of the light and dark bands. Next to the unnumbered chromosome that is most similar to chromosome 1, write 1.
3. Repeat step 2 for chromosomes 2 through 23.

4. Use scissors to cut out all the chromosomes from Figure 2. Tape them in their appropriate places in Figure 3. Note any chromosomal abnormalities. CAUTION: Be careful when handling sharp instruments.

5. Observe the karyotypes in Figures 4 and 5. Note the presence of any chromosomal abnormalities.

6. Draw a data table in which to record your observations of the karyotypes shown in Figures 1, 3,4, and 5. Record any evidence of chromosomal abnormalities present in each karyotype. Record the genetic defect, associated with each type of chromosomal abnormality present. (use the text books or internet to associate a genetic defect with each type of chromosomal abnormality.

Analysis and Conclusions
1. Comparing and Contrasting: Of the four karyotypes that you observed, which was normal? Which showed evidence of an extra chromosome? An absent chromosome?
2. Formulating Hypothesis: What chromosomal abnormality appears in the karyotype in Figure 4? Can you tell from which parent this abnormality originated? Explain your answer.
3. Inferring: Are chromosomal abnormalities such as the ones shown confined only to certain parts of the body? Explain your answer.
4. Drawing Conclusions: Are genetic defects associated with abnormalities of autosomes or of sex chromosomes? Explain your answer.
5. Posing Questions: Formulate a question that could be answered by observing chromosomes of different species of animals.

Going Further
Using library materials or the Internet, research one type of deletion syndrome (a syndrome that results from loss of parts of chromosomes). Write a short paragraph describing the chromosomal abnormality involved and the characteristics of the disorder.