Prelab Preparation

Use the copy masters provided to make appropriate numbers of copies of student worksheets and chromosome patterns for your classes.

Teacher Tips

• Doing these karyotypes is not as easy as it first appears. Give students enough time to work and provide help if they get frustrated. To make it somewhat easier, the 22 autosomes have been repeated in all the chromosome smears in this simulation. Students may struggle with the first karyotype but find successive ones easier as they recognize banding patterns on the chromosomes. (This repeat, of course, does not occur in real karyotypes.) Have students start by completing the karyotype for the normal male. This is a good practice individual.

• All materials are reusable.
• When giving directions to students, you might suggest that they proceed in this fashion:
  1. Count the chromosomes on the worksheet and determine the total number before starting the karyotype.
  2. Cut them out and arrange them from largest to smallest. (That is how they will be arranged on the worksheet.)
  3. Use the banding patterns and the clues on the worksheet to match the homologous pairs.
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  4. Find the sex chromosomes and identify any extra or missing chromosomes.

Discussion

Human karyotypes are usually prepared from photographs of chromosome spreads which have been fixed and stained to highlight banding patterns. The chromosomes in the photograph are cut apart and then arranged in homologous pairs just as students do in this activity.

Chromosomes are easiest to study in their condensed form and, therefore, are obtained from actively dividing cells. White blood cells are used most frequently because they are easily induced to divide and grow in culture. Various chemicals are added to stop the cell’s growth during prophase and metaphase. The cells are placed on microscope slides and treated so they swell and their chromosomes spread apart. Then various stains are used to highlight banding patterns. The treated chromosomes can then be photographed, enlarged if desired, and cut out to do a karyotype.

Once a karyotype has been completed, experienced geneticists who view karyotypes regularly can often pick up potential chromosomal problems. Missing pieces of chromosomes, missing chromosomes, or extra chromosomes can often be detected. The consequences of chromosomal abnormalities can vary from very severe to very insignificant. The karyotypes illustrated in this kit show several common chromosomal patterns. You may want to discuss each syndrome as the karyotypes are completed. Each is described briefly:

1. Normal males have 46 chromosomes (22 pairs of autosomes, plus one X chromosome and one Y chromosome). Because the X and Y are homologs, male sperm contains either the X or Y chromosome and thus determines the sex of the offspring. Because normal males have only one X chromosome (from their mother), there are many genetic patterns (often termed sex-linked) associated with this chromosome.
2. Normal females have 46 chromosomes (22 pairs of autosomes, plus two X chromosomes—one from each parent). Inherited traits on the X chromosomes often show the genetic pattern associated with sex-linked inheritance.
3. Down syndrome (often called trisomy 21) is caused in most cases by the presence of an extra chromosome 21. This will become obvious in the karyotyping of both Down syndrome male and female individuals. Phenotypic consequences of trisomy 21 can vary a great deal. Most individuals with Down syndrome live to adulthood and nearly 80 percent live into their 50s. Down syndrome is the most common serious birth defect in the United States, occurring approximately once in every 700 live births. Mothers of age 35 and older have an increased probability of giving birth to infants with Down syndrome. As a result, many pregnant women now choose to have the karyotype of the fetus checked for trisomy 21 and other chromosomal abnormalities.
4. Klinefelter syndrome is an abnormality associated with the presence of an extra X chromosome in a male. The person has 22 autosomes plus three sex chromosomes XXY. This condition is the result of non-disjunction (failure of homologous pairs to separate during meiosis in gamete production). Males with this disorder are often tall, sexually undeveloped and may have slight intellectual impairment. Recognition of this syndrome before puberty usually doesn’t occur. Many males with this syndrome function well within society and are not diagnosed until they are assessed for infertility as adults.
5. Females with Turner syndrome are missing one copy of the X chromosomes (or some partial deletions of an X chromosome). They have 22 pairs of autosomes and only one X sex chromosome. Individuals are usually short, underdeveloped and sterile. Females can also be XXX genotype, which results in limited fertility and slight intellectual impairment. Women with Turner syndrome usually function well in society.

Teacher Handouts

10147_Teacher1.pdf

Introduction

A karyotype is a display of human chromosomes arranged in homologous pairs. In this activity, you will simulate karyotypes for various individuals and analyze their chromosome patterns.

Concepts

• Homologous chromosomes

• Centromeres
• Chromosomal abnormalities
• Karyotyping
• DNA banding

Materials

Procedure

The instructions on the student worksheet are as follows:

1. Use scissors to cut out the chromosomes for the individual shown on the worksheet.
2. Match the homologous pairs of chromosomes and place them on the worksheet on their appropriate space. Use the line provided to line up the centromere region on each chromosome. Use the shape, size, banding patterns and group descriptions to arrange the karyotype.
3. When the chromosomes have been classified and you are confident of their identity tape them to your worksheet and answer the questions on the worksheet.

Student Worksheet PDF

10147_Student1.pdf