Laboratory Exercise - Nondisjunction
Introduction
Sex Chromosomes
Normally, females have two X chromosomes and males have one X and one Y chromosome.
Humans have 23 pairs (46 total) of chromosomes. The X and Y chromosomes are sex chromosomes. The other 44 chromosomes are called autosomes.
The presence of a Y chromosome determines sex.
The cross below shows that females produce secondary oocytes that have one X chromosome. Half of the sperm produced by males have an X chromosome and the other half have a Y chromosome.
XX x XY
?
This analysis shows that half of the offspring are expected to be male, half are expected to be female.
Nondisjunction
Nondisjunction occurs when chromosomes fail to "disjoin" during meiosis or mitosis. Select one of the links below to review nondisjunction. These links will open a new browser window.
Nondisjunction during meiosis I
Nondisjunction during meiosis II
Monosomy and Trisomy
Monosomy refers to a condition in which there is one chromosome is missing. It is abbreviated 2N - 1. For example, monosomy X is a condition in which cells have only one X chromosome.
A trisomy has one extra chromosome and is abbreviated 2N + 1. Trisomy 21 is an example of a trisomy in which cells have an extra chromosome 21.
Monosomies and trisomies usually result from nondisjunction during meiosis but can also occur in mitosis. They are more common in meiosis 1 than meiosis 2.
They are generally lethal except monosomy X (female with one X chromosome) and trisomy 21 (Down’s Syndrome).
Affected indivisuals have a distinctive set of physical and mental characteristics called a syndrome. For example, trisomy 21 is Down syndrome.
Some common chromosomal abnormalities are listed below.
| Abnormality | Karyotype |
| Down Syndrome | Trisomy 21 |
| Turner Syndrome | X |
| Triple-X Syndrome | XXX |
| Klinefelter Syndrome | XXY |
| Jacob Syndrome | XYY |
Chromosomal Abnormalities - Nondisjunction
Autosomes
Simulation of Normal Mitosis - Chromosome 21
Build a red chromosome and a yellow chromosome using beads to represent the chromatids and magnets to represent the centromeres. Each magnet will have two beads attached to each end as illustrated below.
The red chromosome will represent the chromosome 21 that a person inherited from their mother. The yellow chromosome will represent the chromosome 21 that they inherited from their father.
Arrange a large piece of string on the bench top to represent a cell. Place the two chromosomes 21 in the cell as they would appear during metaphase.
Place the chromosomes in the cell as they would appear during anaphase.
During telophase, the cell begins to pinch into two. Show this by arranging the string to form two partially-divided cells.
Show how the chromosomes are arranged in the cell during telophase.
Draw the two chromosomes during each phase listed in the diagram below. Use two different colors to represent the two different chromosomes.
Figure 1: Normal Mitosis Showing Chromosome 21
Prophase 
Metaphase 
Anaphase 
Telophase 
G1 Interphase 
Indicate the number of chromosomes in each daughter cell below.
Normal human cells have a total of 46 chromosomes. State the total number of chromosomes in cell A (on left) ________ Cell B (right) _____ [Hint: Are they normal human cells?]
Number of chromosomes 21 in cell A (left) _____ Cell B (right) _____ [Hint: Normal cells are diploid, therefore have 2 chromosomes 21.]
Simulation of Nondisjunction During Mitosis - Chromosome 21
Arrange the model cell and two chromosomes as you had for metaphase above.
Show nondisjunction of one of the chromosomes during anaphase.
Show telophase after nondisjunction has occurred. At this point, the chromosome that failed to disjoin earlier has split. It is now two separate chromosomes.
Draw your model of the cell during each of the phases shown in the diagram below. Be sure that your diagram of anaphase shows that the disjoined chromosome has split. There should be three separate single-chromatid chromosomes on one side of the cell.
Figure 2: Nondisjunction of Chromosome 21 During Mitosis
Prophase Metaphase Anaphase Telophase G1 Interphase Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B (right) _____ [Hint: Normal cells have 46 chromosomes.]
Number of chromosomes 21 in cell A (left) _____ Cell B (right) _____ [Hint: Normal cells are diploid, therefore have 2 chromosomes 21.]
How many chromosome 21s are in each of the two daughter cells that result from this nondisjunction? The cell with only one chromosome 21 will not survive. The other cell may survive and continue to divide.
The process that you have just modeled shows how Down syndrome cells can arise as a normal fertilized egg grows and divides by mitosis.
Simulation of Normal Meiosis (Oogenesis or Spermatogenesis) - Chromosome 21
Arrange a piece of string on the desk to represent a cell and place the two chromosomes 21 in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I.
Arrange the cell and chromosomes to represent telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I. There should be one chromosome in each daughter cell.
Show metaphase II, anaphase II, and telophase II for each of these cells.
Draw each of these phases in the spaces below. The bottom row of the table will have two cells in each space.
Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 3: Normal Meiosis Showing Chromosome 21
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____ (normal gametes have 23)
Number of chromosomes 21 in cell A (left) _____ Cell B _____ Cell C _____ Cell D _____
Simulation of Nondisjunction During Meiosis I - Chromosome 21
Nondisjunction during meiosis I
Arrange a piece of string on the desk to represent a cell and place the two chromosomes 21 in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I after nondisjunction has occurred.
Arrange the cell and chromosomes to represent telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I.
Show metaphase II, anaphase II, and telophase II for each of these cells.
Draw each of these phases in the spaces below. The bottom row of the table will have two cells in each space.
Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 4: Nondisjunction of Chromosome 21 During Meiosis I
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____ (normal gametes have 23)
Number of chromosomes 21 in cell A (left) _____ Cell B _____ Cell C _____ Cell D _____
Simulation of Nondisjunction During Meiosis II - Chromosome 21
Nondisjunction during meiosis II
Arrange a piece of string on the desk to represent a cell and place the two chromosomes 21in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I followed by telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I.
Show metaphase II.
Show nondisjunction in one of the cells during anaphase II. The other cell is normal. The chromosome that failed to disjoin eventually does split, forming two chromosomes. Show this during anaphase II.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 5: Nondisjunction of Chromosome 21 During Meiosis 2
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____ (normal gametes have 23)
Number of chromosomes 21 in cell A (left) _____ Cell B _____ Cell C _____ Cell D _____
List the number of chromosomes 21 in the sperm produced by normal meiosis.
List the number of chromosomes 21 in the secondary oocytes produced by nondisjunction during meiosis I. Refer to your diagrams.
List all of the possible kinds of offspring that can be produced if a normal sperm fertilizes secondary oocytes produced by nondisjunction during meiosis I. (See your answers to the two questions above.)
List the number of chromosomes 21 in sperm produced by nondisjunction during meiosis II.
List the number of chromosomes 21 in normal secondary oocytes.
List all of the possible kinds of offspring that can be produced if normal secondary oocytes are fertilized by sperm produced by nondisjunction during meiosis II. (See your answers to the two questions above.)
Sex Chromosomes
Simulation of Normal Oogenesis
Construct two X chromosomes by placing two beads above the centromere and seven beads below the centromere. One chromosome will be composed of red beads and the other will be composed of yellow beads. Each chromosome should have two chromatids as shown below.
Arrange a piece of string on the desk to represent a cell.
Place the two X chromosomes in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I and then telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I. There should be one X chromosome in each daughter cell.
Show metaphase II, anaphase II, and telophase II for each of these cells.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 6: Normal Meiosis in Females, Showing the Sex Chromosomes
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number of X chromosomes in cell A (left) _____ Cell B _____ Cell C _____ Cell D _____
Simulation of Oogenesis with Nondisjunction of the X Chromosomes During Meiosis I
Nondisjunction during meiosis I
Arrange a piece of string on the desk to represent a cell and place the two X chromosomes in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I after nondisjunction has occurred.
Show the cell during telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I.
Show metaphase II, anaphase II, and telophase II for each of these cells.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 7: Nondisjunction of the X Chromosomes During Meiosis I in Females
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number of X chromosomes in cell A (left) _____ Cell B _____ Cell C _____ Cell D _____
Simulation of Oogenesis with Nondisjunction of the X Chromosomes During Meiosis II
Nondisjunction during meiosis II
Arrange a piece of string on the desk to represent a cell and place the two X chromosomes in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I and then telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I. There should be one X chromosome in each daughter cell.
Show metaphase II.
Show anaphase II after nondisjunction in one of the cells. The other cell is normal. The chromosome that failed to disjoin eventually does split, forming two chromosomes. Show this during anaphase II.
Show telophase II.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 8: Nondisjunction of the X Chromosomes During Meiosis 2 in Females
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number of X chromosomes in cell A (left) _____ Cell B _____ Cell C _____ Cell D _____
Simulation of Normal Spermatogenesis
Construct a Y chromosome with yellow beads so that it has one bead above the centromere and two beads below the centromere. The chromosome should have two chromatids.
Arrange a piece of string on the desk to represent a cell.
Place the an X chromosome and a Y chromosome in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I and then telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I. There should be one X chromosome in one daughter cell and one Y chromosome in the other..
Show metaphase II, anaphase II, and telophase II for each of these cells.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 9: Normal Meiosis in Males, Showing the Sex Chromosomes
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Double-check your drawings above to be sure that the X chromosome is longer than the Y chromosome.
Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number and type of sex chromosomes (X or Y) in cell A (left) _____X and _____Y Cell B _____X and _____Y Cell C _____X and _____Y Cell D _____X and _____Y
Simulation of Spermatogenesis with Nondisjunction of the Sex Chromosomes During Meiosis I
Nondisjunction during meiosis I
Arrange a piece of string on the desk to represent a cell and place an X chromosome and a Y chromosome in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I after nondisjunction has occurred.
Show the cell during telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I.
Show metaphase II, anaphase II, and telophase II for each of these cells.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 10: Nondisjunction of the Sex Chromosomes During Meiosis 1 in Males
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Double-check your drawings above to be sure that the X chromosome is longer than the Y chromosome.
Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number and type of sex chromosomes (X or Y) in cell A (left) _____X and _____Y Cell B _____X and _____Y Cell C _____X and _____Y Cell D _____X and _____Y
Simulation of Spermatogenesis with Nondisjunction of the Y Chromosome During Meiosis II
Nondisjunction during meiosis II
Arrange a piece of string on the desk to represent a cell and place an X chromosome and a Y chromosome in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I and then telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I.
There should be a Y chromosome in one of the daughter cells and an X chromosome in the other.
Show metaphase II.
Show anaphase II after nondisjunction in the cell that contains the Y chromosome. The other cell is normal. The chromosome that failed to disjoin eventually does split, forming two chromosomes. Show this during anaphase II.
Show telophase II.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 11: Nondisjunction of the Y Chromosome During Meiosis 2 in Males
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Double-check your drawings above to be sure that the X chromosome is longer than the Y chromosome.
Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number and type of sex chromosomes (X or Y) in cell A (left) _____X and _____Y Cell B _____X and _____Y Cell C _____X and _____Y Cell D _____X and _____Y
Simulation of Spermatogenesis with Nondisjunction of the X Chromosome During Meiosis II
Nondisjunction during meiosis II
Arrange a piece of string on the desk to represent a cell and place an X chromosome and a Y chromosome in the cell as they would appear during metaphase I of meiosis.
Next, arrange the chromosomes to represent anaphase I and then telophase I.
Use two smaller pieces of string to represent the two daughter cells formed after telophase I.
There should be a Y chromosome in one of the daughter cells and an X chromosome in the other.
Show metaphase II.
Show anaphase II after nondisjunction in the cell that contains the X chromosome. The other cell is normal. The chromosome that failed to disjoin eventually does split, forming two chromosomes. Show this during anaphase II.
Show telophase II.
Draw each of these phases in the spaces below. Recall that homologous chromosomes are paired during prophase I and remain paired during metaphase I. Your drawings should reflect this pairing.
Figure 12: Nondisjunction of the X Chromosome During Meiosis 2 in Males
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Haploid Daughter cells Double-check your drawings above to be sure that the X chromosome is longer than the Y chromosome.
Indicate the number of chromosomes in each daughter cell below.
Total number of chromosomes in cell A (on left) ________ Cell B _____ Cell C _____ Cell D _____
Number and type of sex chromosomes (X or Y) in cell A (left) _____X and _____Y Cell B _____X and _____Y Cell C _____X and _____Y Cell D _____X and _____Y
Some Genetic Crosses Involving Nondisjunction
Use the information above to answer the questions below.
List all of the kinds of sperm that can be produced as a result of nondisjunction of the sex chromosomes during meiosis I. The answer to this question can be obtained from your drawings (Figure 10).
Use a Punnett square analysis to show the kinds of offspring produced when normal female gametes (X) are combined with sperm produced with nondisjunction of the sex chromosomes during meiosis I. One side of your Punnett square will contain normal female gametes (all X) and the other side will contain the sperm listed in the previous question.
Name the abnormalities produced in the above cross.
List all of the kinds of sperm that can be produced as a result of nondisjunction of the X chromosomes during meiosis II. The answer to this question can be obtained from your drawings (Figure 12).
Use a Punnett square analysis to show the kinds of offspring produced when normal female gametes are combined with sperm produced with nondisjunction of the X chromosome during meiosis II. One side of your Punnett square will contain normal female gametes (all X) and the other side will contain the sperm listed in the previous question.
Name the abnormalities produced in the above cross..
List all of the kinds of sperm that can be produced as a result of nondisjunction of the Y chromosomes during meiosis II. The answer to this question can be obtained from your drawings (Figure 11).
Use a Punnett square analysis to show the kinds of offspring produced when normal female gametes (X) are combined with sperm produced with nondisjunction of the Y chromosome during meiosis II. One side of your Punnett square will contain normal female gametes (all X) and the other side will contain the sperm listed in the previous question.
Name the abnormalities produced in the above cross..
List all of the kinds of female gametes that can be produced as a result of nondisjunction during meiosis I. The answer to this question can be obtained from your drawings (Figure 7).
Use a Punnett square analysis to show the kinds of offspring produced when normal sperm (X or Y) are combined with female gametes produced with nondisjunction of the X chromosomes during meiosis I. One side of your Punnett square will contain normal sperm (X and Y) and the other side will contain the gametes listed in the previous question.
Name the abnormalities produced in the above cross..
List all of the kinds of secondary oocytes that can be produced as a result of nondisjunction during meiosis II. The answer to this question can be obtained from your drawings (Figure 8).
Use a Punnett square analysis to show the kinds of offspring produced when normal sperm (X or Y) are combined with female gametes produced with a nondisjunction of one of the X chromosomes during meiosis II. One side of your Punnett square will contain normal sperm (X and Y) and the other side will contain the gametes listed in the previous question.
Name the abnormalities produced in the above cross.
