Magnetic Karyotypes

Introduction

Teach students to identify and classify chromosome smears according to the Denver System using large magnetic demonstration models or pictograms of chromosomes. Students will investigate ten different scenarios involving both normal karyotypes as well as those which exhibit chromosomal abnormalities.

Concepts

• Chromosome smear
• Chromosomal abnormalities
• Karyotyping
• Homologous chromosomes

Background

Normal human somatic (body) cells have 46 chromosomes. The 46 chromosomes include two possible types of sex chromosomes, X and Y, and 22 pair of autosomes. One copy of each autosome and an X chromosome inherited from the mother. The father provides a second copy of each autosome plus either an X or Y sex chromosome. An embryo that receives an X chromosome from the father will be female, while an embryo that receives a Y chromosome from the father will be male. The autosomes pair up in so-called homologous or matching chromosomes. After a chromosome smear (slide preparation) has been stained, each pair of homologous chromosomes is easily distinguished from other chromosomes by differences in length, the position of the centromere, and the pattern of bands which are visible by the application of certain stains.

The centromere is always found in one of three locations in human chromosomes (see Figure 1). If the centromere is located in the center of the chromosome, it is called metacentric. If the centromere is located near one end of the chromosome, it is called acrocentric. Finally, if the centromere is located between the center of the chromosome and the end of the chromosome, the chromosome is called submetacentric. The two ends of the chromosome are called the p arm (shorter arm) and the q arm (longer arm).

In order to facilitate comparison of the genetic makeup of people from all over the world, geneticists established a classification and naming system, called the Denver System, to describe chromosomes. The Denver System was established in 1961 at an international meeting of geneticists in Denver, CO. According to this system, the sex chromosomes are named X and Y, while the autosomes are numbered in descending order, with the largest called chromosome 1 and the smallest called chromosome 22. The Denver System further subdivides or classifies the chromosomes into eight groups A–G (see Table 1). A karyotype is the specific arrangement of specially stained chromosomes obtained using the Denver System.

Problems occurring during mitosis and meiosis may result in cells containing too many or too few chromosomes or parts of chromosomes. Other problems may occur if part of one chromosome breaks off and becomes attached to a different chromosome. The consequences of chromosomal abnormalities can vary from insignificant to fatal, depending on the size and location of the error and when the error occurred. For example, an embryo with three copies of chromosome 1 will result in a miscarriage, whereas cancer may occur when specific genes are deleted from chromosomes during mitosis in an adult.

There are two basic types of genetic abnormalities that can be detected by geneticists using a karyotype—numerical errors and structural errors (see Figure 2). Numerical errors include trisomy, in which three copies of a chromosome are present instead of two. Monosomy is a numerical error in which only one copy of a particular chromosome is present. Structural errors occur when part of a chromosome is missing or not located in its correct position. There are four types of structural errors—translocations, inversions, deletions and duplications. Translocations arise when part of one chromosome breaks off and attaches to another chromosome. Chromosomes with acrocentric centromeres (centromere near one end), such as chromosomes 13, 14, 15, 21 and 22 have very short p arms that break off rather easily. The remaining long q arm may translocate and stick to another acrocentric chromosome (see Figure 2). Inversions involve a section of a chromosome breaking off and then reattaching to the same chromosome upside down. If a section of a chromosome is completely absent it is called a deletion. Duplication occurs when a section of chromosome is repeated.

In some cases a chromosomal abnormality may be present in the gamete (egg or sperm cell) due to problems occurring during meiosis. A faulty gamete will produce an embryo in which every cell contains the abnormality. Full numerical abnormalities are usually fatal and the embryo miscarries. The few exceptions are Monosomy X (one X), Disomy Y (two Ys), and Trisomy 13, 18, 21 or X (three copies of the same chromosome). Surviving structural abnormalities are not uncommon because the extent of the change from “normal” varies.

Mosaicism occurs if the chromosome abnormality occurs during mitosis in the embryo. A mosaic is an embryo with two different genotypes—some of the cells have a normal set of chromosomes, while others will have the abnormal set of chromosomes. The severity of abnormality in a mosaic can be mild or severe depending upon how old the embryo is when the error occurs and how much of the chromosome is in error. If the error occurs late in development, very few cells will carry the error and the individual will have mostly normal-functioning cells with either mild or no problems. If the error occurs early in development then a majority of the cells will carry the error, resulting in potentially severe problems such as mental retardation and heart defects.

The karyotypes of a normal male and female, as well as individuals with different numerical errors, are provided in this activity. Table 2 summarizes the genetic composition of the different karyotypes that will be classified in this activity. The genetic composition of the karyotype is summarized as follows: first, by the number of chromosomes present in the cell (45, 46 or 47); then by the sex chromosomes present (XX or XY); and finally by the number of abnormal chromosome (e.g., 21, X). For example, 47, XXY is a male with two X chromosomes and one Y chromosome rather than the usual one X and one Y.

Materials

Safety Precautions

This activity is considered nonhazardous.

Disposal

All materials used in this activity may be stored for future use.

Prelab Preparation

1. Cut out each chromosome from the magnetic sheet. Tip: Use a paper cutter for the major cuts followed by using scissors for the more intricate cuts.
2. Obtain a copy of the Denver System Template. Using a dry erase marker or chalk to copy the template onto the magnetic surface where the demonstration will be performed.
3. Obtain two copies of chromosomes 1–22 (autosomes) as well as one X sex chromosome and a Y sex chromosome.
4. Scramble the 46 chromosomes in a random pattern on the magnetic board (see Figure 3).

Procedure

1. Count the number of chromosomes present in the chromosome smear and record the results on the board.
2. Arrange the chromosomes in order of decreasing size, from largest to smallest.
3. Use the size, centromere location and banding pattern on each chromosome to match homologous pairs of chromosomes.
4. Place the matching pairs on the Denver System Template on the board with the centromere located on the line provided and the short p arm above the line.
5. Have the class identify the karyotype as representing a normal male.
6. Make the following modifications to the karyotype of a normal male to demonstrate a normal female karyotype as well as various chromosome abnormalities that may occur. Note that the syndrome and sex of the example are listed in bold and additional information is in italics.

Normal Female (only ♀)

7. Remove the Y sex chromosome from this karyotype.
8. Replace it with a second X chromosome to identify normal female consists of 22 pairs of autosomes and 2 X sex chromosomes.

Edwards Syndrome—Female (Three times more common in ♀ than ♂)

9. Obtain a third copy of chromosome 18 and place it next to the homologous pair of chromosome 18.
10. The Edwards Syndrome female has two copies of autosomes 1–17 and 19–22, two X sex chromosomes, and three copies of chromosome 18.

Triple X Syndrome—Female (only found in ♀)

11. Remove the third copy of chromosome 18. Obtain a third copy of an X sex chromosome and place it next to the two existing X sex chromosomes.
12. The karyotype should display two copies of autosomes 1–22 and three X sex chromosomes.

Turner Syndrome—Female (only found in ♀)

13. Remove two X sex chromosomes.
14. The female karyotype has two copies of each autosome, 1–22 and one X sex chromosome.

Robertsonian Translocation—Female (found in ♂ and ♀)

15. Add a second X sex chromosome and remove both copies of chromosome 21. Replace the chromosome 21 homologues with translocation 21 and 21 chromosome.
16. The karyotype has two copies of autosomes 1–20 and 22, two copies of chromosome 21 that are attached at the p arms to form one chromosome as well as two X sex chromosomes.

Patau Syndrome (Trisomy 13)—Male (found in ♂ and ♀)

17. Remove the translocation 21 and 21 chromosome and replace it with chromosome 21 homologues. Add a third copy of chromosome 13.
18. The Patau Syndrome karyotype should have two copies each of autosomes 1–12 and 14–22 and three copies of autosome 13. The example is male so it also has an X and a Y sex chromosome.

Klinefelter Syndrome—Male (only ♂)

19. Remove the third copy of chromosome 13 and add an extra X sex chromosome.
20. There should be two copies of each autosome, 1–22 and two X sex chromosomes and one Y sex chromosome.

Down Syndrome (Trisomy 21)—Male (found in ♂and ♀)

21. Remove one X sex chromosome and add a third copy of chromosome 21.
22. This karyotype will have two copies each of autosomes 1–20 and 22 and three copies of autosome 21. Since the example is male, it will have an X sex chromosome and a Y sex chromosome.

Disomy Y (47,XYY)—Male (found only in ♂)

23. Remove the third copy of chromosome 21 and add a second Y sex chromosome.

Student Worksheet PDF

10981_Student1.pdf

Teacher Tips

• Read and discuss the Background section as a class before beginning this activity.
• Chromosome 21 actually contains fewer base pairs and is therefore shorter than chromosome 22. This was not known in 1961 when the Denver System was established.
• Provide students with reference pictures to reference of characteristic features genetic abnormalities associated with different chromosome errors so they can see the produced phenotype.
• Flinn Scientific also offers student laboratory activities studying karyotyping. See Karyotyping with Ideograms, Catalog No. FB1858 and Human Karyotyping Kit, Catalog No. FB1111.

Answers to Questions

1. What is the genetic composition of a normal female?

46, XX

2. Why must chromosomes be stained before they are organized according to the Denver System?

Chromosomes must first be stained so that the bands are visible. The bands must be visible to accurately match a chromosome to its homologous pair.

3. After a chromosome smear has been stained, what are the next steps in organizing chromosomes according to the Denver System?

After chromosome smears are stained, the total number of chromosomes should be counted. The next step is to arrange them according to height followed by matching their banding patterns to form homologous chromosomes.

4. What chromosomal abnormalities might be disguised phenotypically?

Triple X syndrome sometimes does not show any phenotypical symptoms, and Disomy Y may sometimes be hidden depending on the time in development the second Y was produced.

5. Individuals with Down Syndrome (Trisomy 21) may experience different degrees of health symptoms. Explain why this may occur.

One person may have Down Syndrome in all their cells while the other is a mosaic. Depending on the time in development the error occurred may cause the mosaic individual to exhibit several or very few of the characteristic traits.

6. Miscarriages may occur for different reasons. Explain why an apparently normal female with a Robertsonian Translocation may experience frequent miscarriages. Explain in terms of genotype and phenotype.

Women who have a Robertsonian Translocation are often phenotypically normal but unbalanced gametes lead to miscarriages.

References

Michelle Barnet, “A Magnetic Karyotype,” The Nucleus Newsletter, Texas Association of Biology Teachers, Winter 2006.

Online Resources (accessed July 2009)
http://ghr.nlm.nih.gov/
http://marchofdimes.com
http://medlineplus.gov/