Laboratory Review ❮ 247Key Skill- Analyze data using chi-square analysis. You can be sure that the AP exam will have at least
one question asking you to do this. This lab is excellent practice for such a question.
Part 2: Meiosis and Crossover inSordaria
Basic Setup and Results for Part 2
The title to this section makes Sordariasound like some posh vacation spot in Europe. In
reality it is a haploid ascomycete fungus. Anyway, the final portion of this experiment looks
at the crossover that occurs during meiosis of this fungus and briefly discusses how recom-
bination maps can be created using such data. Meiosis in Sordariaresults in the formation
of eight haploid ascospores, each of which can develop into a new haploid fungus.
Crossover in Sordariacan be observed by making hybrids between wild-type and mutant
strains. Wild-type Sordariahave black ascospores, and mutants have different colored
ascospores (e.g., tan). When mycelia of these two strains come together and undergo meio-
sis, and if no crossover occurs, the asci that develop will contain four black and four tan
ascospores in a 4:4 pattern. If crossover occurs, the ratio will change to either 2:2:2:2
or 2:4:2.
Chapter 10, Heredity, discusses gene maps constructed from crossover frequencies.
You would construct the map here by first determining the percentage of asci that showed
crossover. Referring to Figure 19.3, count the number of 2:2:2:2 and 2:4:2 asci and divide
that sum into the total number of offspring. This result multiplied by 100 will give the
crossover percentage. This number can then be used to determine how far away the gene is
from the centromere. The crossover percentage is divided by 2 to determine this distance
because a crossover involves only half the spores in each ascus.Key Skill- Explain how meiosis and crossing over leads to increased genetic diversity.
Wild typeMutant4:4 2:2:2:2 2:2:2:2 2:4:2 2:4:2Crossing over has occurred.Figure 19.3 Crossover patterns in Sordaria.KEY IDEA
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