Crossing Over Lab Activity

Genetic diversity is a very important part of evolution. Without different genetics available in the gene pool, species would not be able to adapt to an ever-changing environment and evolve to survive as those changes happen. Statistically, there is no one in the world with your exact same combination of DNA (unless you are an identical twin). This makes you unique.

There are several mechanisms that contribute to the large amounts of genetic diversity of humans, and all species, on Earth. Independent assortment of chromosomes during Metaphase I in Meiosis I and random fertilization (meaning, which gamete fuses with a mate's gamete during fertilization is randomly selected) are two ways your genetics can be mixed during the formation of your gametes. This ensures that every gamete you produce is different from all of the other gametes you produce.

What Is Crossing Over?

Another way to increase genetic diversity within an individual's gametes is a process called crossing over. During Prophase I in Meiosis I, homologous pairs of chromosomes come together and may exchange genetic information. While this process is sometimes difficult for students to grasp and visualize, it is easy to model using common supplies found in pretty much every classroom or home. The following lab procedure and analysis questions can be used to help those struggling to grasp this idea.

Materials

• 2 different colors of paper
• Scissors
• Ruler
• Glue/Tape/Staples/Another attachment method
• Pencil/Pen/Another writing utensil

Procedure

1. Choose two different colors of paper and cut two strips out of each color that are 15 cm long and 3 cm wide. Each strip is a sister chromatid.
2. Place the strips of the same color across each other so they both make an “X” shape. Secure them in place with glue, tape, staple, a brass fastener, or another method of attachment. You have now made two chromosomes (each “X” is a different chromosome).
3. On the top “legs” of one of the chromosomes, write the capital letter “B” about 1 cm from the end on each of the sister chromatids.
4. Measure 2 cm from your capital “B” and then write a capital “A” at that point on each of the sister chromatids of that chromosome.
5. On the other colored chromosome on the top “legs”, write a lowercase “b” 1 cm from the end of each of the sister chromatids.
6. Measure 2 cm from your lower case “b” and then write a lower case “a” at that point on each of the sister chromatids of that chromosome.
7. Place one sister chromatid of one of the chromosomes over the sister chromatid over the other colored chromosome so that the letter “B” and “b” has crossed over. Be sure the “crossing over” occurs between your “A”s and “B”s.
8. Carefully tear or cut the sister chromatids that have crossed over so that you have removed your letter “B” or “b” from those sister chromatids.
9. Use tape, glue, staples, or another attachment method to “swap” the ends of the sister chromatids (so you now end up with a small part of the different colored chromosome attached to the original chromosome).
10. Use your model and prior knowledge about crossing over and meiosis to answer the following questions.

Analysis Questions

1. What is “crossing over”?
2. What is the purpose of “crossing over”?
3. When is the only time crossing over can occur?
4. What does each letter on your model represent?
5. Write down what letter combinations were on each of the 4 sister chromatids before crossing over happened. How many total DIFFERENT combinations did you have?
6. Write down what letter combinations were on each of the 4 sister chromatids before crossing over happened. How many total DIFFERENT combinations did you have?
7. Compare your answers to number 5 and number 6. Which showed the most genetic diversity and why?