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.
How can chromosomes be observed?
If there are chromosomal abnormalities, how do they affect you as a person?
Read the entire investigation. Then work with a partner to answer the pre lab questions on the lab answer key.
Materials (per student)
scissors glue or transparent tape
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. Mentally 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 at the end of this lab. This karyotype smear was obtained by staining and photographing a cell in mitosis.
2. You are going to make a karyotype of these chromosomes. Cut out each chromosome. Do not lose any. Carefully note the size of the chromosome, the position of the the centromere and the stain pattern of light and dark bands. Match up all the homologous pairs. Keep in mind that if this is a male two chromosomes WON’T match.
3. Arrange the homologous pairs from longest pair to smallest pair. The longestt pair of chromosomes is designated as chromosome one, the next largest is chromosome number two, and so on and so forth. Once you have the chromosomes identified glue them onto figure three in their proper places. Hint: Chromosomes #13, 14, 15, 21, and 22 have satellites.
4. Note any chromosomal abnormalities and identify such by placing the chromosome number and the sex of the person on the bottom right corner of the sheet. See Normal Male example in figure one
5. Observe the karyotypes in Figures 4 and 5. Note the presence of any chromosomal abnormalities.
Karyotype Lab Answer Key
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?
POST LAB QUESTIONS
6. Draw a data table in the space below 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. Research genetic defects in the following chapters and record the genetic defect of each trait associated with each type of chromosomal abnormality present.
7. Comparing and Contrasting Of the four karyotypes that you observed, which was normal? Which showed evidence of an extra chromosome? An absent chromosome?
8. Formulating Hypotheses What chromosomal abnormality appears in the karyotype in Figure 4? Can you tell from which parent this abnormality originated? Explain your answer.
9. Inferring Are chromosomal abnormalities such as the ones shown confined only to certain parts of the body? Explain your answer.
10. Drawing Conclusions Are genetic defects associated with abnormalities of autosomes or of sex chromosomes? Explain your answer.
11. Posing Questions Formulate a question that could be answered by observing chromosomes of different species of animals.