Why it is important to know the locations of genes in the genome.

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Drosophila Three-Point Test Cross Lab Write-Up Instructions (65 points)

Abstract (5 points)

In a short paragraph describe the experiment that was done as well as the major findings. Clarity is essential. The abstract is usually written last and is limited to 200 words.

Introduction (14 points)

Provide ALL background information a reader would need to understand the purpose, results and analysis of the experiment. Must include:

1. Why it is important to know the locations of genes in the genome.
2. A description a crossing-over during meiosis, linkage, recombination frequency (RF), and how RF relates to map units?  How does RF change for closely linked versus distantly linked and unlinked, genes?
3. Why is it advantageous to map three (or more) genes at once instead of mapping each pair of genes separately
4. The benefits of Drosophila for genetic analyses
5. The hypotheses for this experiment
• RF measured in lab will be similar to the expected RF based on known map distances.
• Reciprocal classes will occur and survive in equal numbers.
• Interference will be a positive value.

 

Methods (14 points)

This section should provide enough information so that the reader could carry out the experiment independently.

1. Explain the experimental strategy: P, F1 and F2.  Describe all genotypes and expected phenotypes.
2. Describe the different traits that were scored.
3. Why was it unnecessary to determine the sex of the F2?
4. Describe calculations for RF, map units, and Interference.
5. Describe the Chi-Square tests that were done and the highest acceptable Chi-Square value for a corresponding p value of 0.05 or lower for relevant degrees of freedom used in your different Chi-Square tests.  (Measured vs. published map distances; reciprocal classes)

 

Results (14 points)

In this section, the data are shown in tables AND explained in coherent paragraphs.

1. Produce a table with the counts of each F2 phenotype for:your group’s data, your lab section’s data, data provided by the fly experts. (Note to TA’s: This lab has a long history of terrible data, so each lab instructor will invent a dataset for each of her/his lab sections. These data sets MUST change each semester!)
2. Produce THREE genetic maps, each based on each of the data sets in the Table. Calculate Interference for each data set. Show the equation for calculation of interference.
3. Compare expected and observed data for pairwise map distances among the three genes and for reciprocal crosses using Chi-Square values. Report p-values for all comparisons, and state whether differences between expected and observed data can be attributed to chance. Do this for all data sets. (There will be 18 Chi-square calculations, 6 per data set.)
4. A narrative must describe the table, mapping calculations and Chi-Square calculations.  You must interpret your Chi-Square results.  Can deviations from expected values be attributed to chance?  Explain your reasoning.

 

Discussion (14 points)

The results are summarized in this section and the reasons WHY data were significantly different than expected are considered.

1. How do map units calculated from the three data sets (one small and two large) compare to published distances?
a. What happened for the shorter y-cv distance?
b. What happened for the longer cv-f distance?
c. What happened with the 4 reciprocal classes?  In the case of reciprocal classes, were any trends observed (certain reciprocals tend to be near equal while others were quite different)? How do mutations affect viability?
d. Did these results match the hypotheses stated earlier?
2. Why is it difficult to accurately measure long map distances by RF?
a. What can be done for more accurate measurements of long map distances?
3. What difficulties arose when assigning phenotypes when scoring the F2?
a. What could be done to reduce these difficulties?

 

Overall Conclusions (4 points)

Keep this section short, one paragraph at the most. Do not repeat yourself over and over when writing this paragraph!

What do the data demonstrate?

Why is a statistical analysis important?

Summarize ways to improve the outcome of the three point testcross mapping experiment; describe “tricks” for evaluating phenotypes.

 
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