1. AP Biology Sordaria Lab Meiosis

2. Sordaria Life Cycle Species of microscopic fungus.
Commonly found in the feces of herbivores
Haploid for most of the life cycle & exists in a mass called the mycelium (“body” of fungus)
Ascospores (haploid)  mycelium  forms fungal filaments (hyphae)  2 hyphae fuse (diploid zygote)  meiosis  ascospores
Spore color gene: black (wild type) or tan (mutant)
Wild type
Mutant

3. Sardoria Perithecium and Asci
Perithecium- fruiting body of the fungus
Ascus- sexual spore-bearing cell, surrounds ascospores
Ascospores- spores (gametes), 8 ascospores/ascus
perithecium
ruptured perithecium
ascus
Perithecium ruptures and
ascus with ascospores are released

4. Ascospore Arrangement
Arrangement of ascospores in ascus can be used to determine if crossing over has occurred during meiosis. Ratios can be used to determine the gene’s distance from the centromere and each other no crossing over 4:4 arrangement crossing over 2:2:2:2 arrangment 2:4:2 arrangement

6. Ascospore Arrangement Patterns

7. No Crossing Over
4 x 4

8. Crossing Over
2, 2, 2, ,4,2

9. Crossing Over and Gene Mapping
Crossover events allow us to approximate distances between genes and map chromosomes
• Theory states that the farther two genes are from one another (or from the centromere), the more likely they will be crossed over to a non-sister chromatid during meiosis
• Conversely, if two genes are close to one another, the likelihood is low that they will crossover, and they will end up on the same chromosome

10. Hypothesis Statements
H0: Genes are NOT linked on the same chromosome (no evidence of crossover exists) Ha: Genes exist together on the same chromosome (evidence of crossover exists)

11. Counting Recombinant Asci
Count all the recombinant asci on the picture cards.
- count only complete asci (ones with 8 ascospores)
Do not count ascospores that are all tan or all black.
Record your results in your table.
Published results: 27 map units
How to identify the asci
non- recombinant recombinant

12. Calculating Color Gene to Centromere Distance
1. Count # Noncrossover Asci # Crossover Asci Total # Asci 2. Calculate recombination frequency # crossovers/ total # 3. Corrected value: % recombination ÷ 2 Mitosis event after meiosis  2 asci formed 4. This is the map distance from gene to centromere

13. Sample Data (tan x wild cross)
Sardoria Table
Sample Data (tan x wild cross)
Number of
Asci not
Showing
Crossover
4:4
Asci
2:2:2:2 Or
2:4:2
Total
Counted %
% Showing
Crossover/2
(only ½ of ascospores are a result of crossing over, other half are just a mitotic copy)
Gene to
Centromere
Distance
(map units)
157
168
325
168/325=0.52
0.52/2=.26
.26x100=26%
27 m.u.
(published data)