1. In an ordered tetrad from a trans heterozygote (a+/+b) there are two basic patterns of nonparental ditypes.

   1. What does "nonparental ditype" mean? 

   2. Draw both nonparental ditype patterns. 

   3. For each pattern in (b) describe the events that must have occurred for the pattern to be generated (be sure to indicate when during meiosis these events happen). 

   4. Which of the two patterns do you expect to see more frequently? Why? 

2. For recessive mutations in two different genes (a and b) the ratio of parental ditype (PD) tetrads to nonparental ditype (NPD) tetrads will be quite different if a and b are linked or if they are unlinked.

1. Define "parental ditype" and "nonparental ditype"

2. What is the relationship (which is bigger, if either) between PD and NPD tetrads if a and b are linked?  Show your derivation.

3. What is the relationship between PD and NPD tetrads if a and b are unlinked?  Show your derivation.

3. Two strains (a and +) of Neurospora are crossed and the 8-spored, linear asci are examined. Some of these are:

1. Define gene conversion and indicate in which of the tetrads had gene conversion taken place? 

2. Which tetrads contained hybrid DNA in the region immediately after meiosis occurred? 

3. In which tetrads was the hybrid DNA not repaired? 

4. Which tetrads would probably not occur in a repair-deficient strain of Neurospora? 

5. Order the tetrads according to the frequency (highest to lowest) that you would expect to find them. Explain your answer (be sure to state any assumptions you need to make). 

4. Answer TRUE or FALSE.  If the statement is TRUE, explain or give a supporting example.  If the statement is FALSE, either correct the statement or give a counterexample.

1. For the average gene loss-of-function alleles are more likely to arise than gain-of-function alleles.

2. For unordered tetrads one need only look at the number of tetratype tetrads to determine the number of double crossovers.

3. Half of all gene conversions are associated with recombination.

4. Half of all recombination events are associated with gene conversion events.

5. Half conversion events occur when DNA polymerase repairs a base mismatch.

6. Gene conversion will be seen twice as frequently as recombination

7. In an ordered tetrad an M_I division pattern indicates that recombination has occurred between the centromere and the gene of interest.

5. Three models were given in class to explain recombination in fungi (Holliday, Meselson-Radding, and Double-stranded Break Repair). 

1. State the essential features of each, paying particular attention to how they differ from each other.

2. State what data they are trying to explain, i.e., why each had to be developed.

6. a. What common mechanism leads to half gene conversion in the Holliday model, the Meselson-Radding model, and the double-strand break repair model?

2. What additional aspect of the Meselson-Radding model (also found in the double-strand break repair model) could also give rise to half gene conversion? 

3. What process leads to full conversion for all three models?

4. If the mechanism in part c. was inactive, how could you (by looking for appropriate octads) determine whether gene conversion utilized the Holliday mechanism or the Meselson-Radding mechanism?

7. Define and distinguish briefly between production of 3:5 octads by the Holliday model vs. production of 3:5 octads by the Meselson-Radding (or double-strand break) model.

8. Explain how the double strand break model resolves to give recombination for one-half of all the gene conversion events.

Answers