LabBench Activity

Analysis of Results II

slide of Sordaria asci

Study this small section of a slide of Sordaria to determine if crossing over has occurred in the asci designated by an X.

If the ascospores are arranged 4 dark/4 light, count the ascus as "No crossing over." If the arrangement of ascospores is in any other combination, count it as "Crossing over." (Keep track of your counts with paper and pencil.)

In this exercise, we are interested only in asci that form when mating occurs between the black-spore strain and the tan-spore strain, so ignore any asci that have all black spores or all tan spores. Occasionally the asci rupture and spores escape. You can see them here as individual spores not in one of the possible arrangements, so don't include them in your count.

1. In the photo, how many asci marked with an X show no evidence of crossing over?

2. In the photo, how many asci marked with an X show evidence of crossing over?

3. In the photo, what is the total number of asci marked with an X?

4. What is the percent of crossovers? (number of asci with crossovers divided by total number of asci multiplied by 100)

By studying the frequency of crossing over, you can gather information that lets you draw a map of the relative location of genes on a chromosome. A map unit is a relative measure of the distance between two linked genes, or between a gene and the centromere. The greater the number of crossovers, the greater the map distance.

From the crossing over data you gather for Sordaria, you will be able to calculate the map distance between the gene for spore color and the centromere. To find the number of map units, you divide the percent of crossovers by 2. (In an actual lab, you should count at least 50 asci before calculating map units.)

5. For the sample shown here, what is the map distance between the gene for spore color and the centromere? (percent of crossovers divided by 2)

Why divide by two? Each crossover produces two spores like the parents and two spores that are a result of the crossover. Thus, to determine the number of crossovers, you must divide the number of asci counted by two since only half the spores in each ascus result from crossing over.