Genetic Diversity: Crossing Over During Meiosis

A population of organisms such as gray squirrels shows genetic diversity: no two animals are exactly alike. One mechanism that contributes to genetic diversity is crossing over during meiosis, cell division that produces eggs and sperm. During meiosis, matching chromosomes from each parent, homologous chromosomes, form pairs or tetrads. The two homologous chromosomes then swap alleles of genes in a process called crossing over. An allele is one of the forms of a gene. Because of this swapping, the cells produced by meiosis contain chromosomes with traits of both parents. In this activity crossing over will be demonstrated in a model.

Materials

Small ball of red modeling clay

Small ball of blue modeling clay

Masking tape

Ruler

Pen

Activity

1. Use the red modeling clay to make a 12cmtube. This represents a chromosome.
2. To show that this chromosome has replicated, make another red clay tube the same length. Connect the two tubes at the center by pinching the clay together. The two parts of a replicated chromosome are called sister chromatids.
3. Show three genes on each of the sister chromatids. Begin as follows:
   1. Place a small piece of masking tape on each of the sister chromatids at the following locations: 2 cm from the top, 4 cm from the top, and 10 cm from the top.
   2. Label the tape from top to bottom as A, B, and C to represent three genes on the chromosome.



4. Use the blue modeling clay and masking tape to make another replicated chromosome to match the red one. The red and blue chromosomes represent a homologous pair.
5. Place the two homologous chromosomes side by side to represent a tetrad.
6. Simulate crossing over by pinching off the blue modeling clay labeled C on one chromatid and swapping it with the red modeling clay labeled C on one chromatid.

Follow-Up Questions

1. What are homologous chromosomes?
2. How many genes were located on each chromosome in this activity?
3. In this simulation, how many genes were exchanged during crossing over?

Answers

1. Homologous chromosomes are matching chromosomes from each parent.
2. Three.
3. One.

Extension

Use your clay models of chromosomes to represent the entire process of meiosis.