This document provides instructions for writing a lab report on a three-point testcross experiment in Drosophila. The report should include an abstract, introduction, methods, results, discussion, and conclusions section. The introduction should provide background on genetic mapping and crossover frequency. The methods should describe the experimental design, scoring, and calculations. The results should present phenotypic counts, genetic maps, and chi-square tests comparing expected and observed values. The discussion should interpret results in light of hypotheses, published data, and difficulties encountered. The conclusions should summarize key findings and ways to improve the experiment.

1. Drosophila
Three-Point Test
C
ross 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
a
bstract 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.
W
hy i
t
i
s
important to know the location
s
of gene
s
in the genome.
2.
A description a crossing-over during meiosis, linkage,

2. recombination frequency (RF),
and how
RF
relate
s
to map units
?
How does RF change for
closely
linked
versus
distantly linked and unlinked
,
genes?
3.
W
hy is it advantage
ous
to map
three (or more)
genes at on
c
e instead of mappin
g each pair of genes separately
4.
T
he benefits
of
Drosophila
for genetic analyses
5.
T
he
hypotheses for this experiment

3. •
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
(1
4
points)
This section should provide enough information so that the
reader could carry out the experiment independently.
1.
Explain the experimental strategy: P, F
1
and F
2
. 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 F
2
?
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

4. 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
F
2
phenoty
pe 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/h
is lab sections.
These data sets MUST change each semester!)
2.
Produce
THREE
genetic maps, each based on

5. 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 distance
s 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

6. 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 F
2
?
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.
PLAGIARISM:
Remember,
you must use your own words, even if you work with others to
discuss what the content of your paper will be. Do not use

7. quotations; read material, figure out what it means, and
then explain in your own words. If you do use material not
found in the lab manual or the textbook, be sure to cite it.
Instructions for citations are found in the oral presentation
section of the Genetics Lab Manual. All papers must be .doc or
.
docx
files, and will be submitted to your lab’s BeachBoard Dropbox
and will be subject to plagiarism detection using
Turnitin
.
A strict ZERO policy (on the entire write-up) will apply to all
p
lagiarism that goes beyond a shared, common phrase.
If two students’ papers are found to be highly similar, BOTH
students will receive a ZERO. Do not give your word file to a
friend to help them out at the last minute; they will likely take
both of you down.
Papers must be uploaded to the lab
BeachBoard
Dropbox BEFORE your lab starts on the designated due date.
Please see
http://philosophy.tamu.edu/~gary/intro/plagiarism.index.html
for some examples of plagiarism.
Citations
To respond to question 1 of the Introduction, you will need to
look up papers. Cite these as described in the group oral
presentation instructions in the Genetics Lab Manual.
Writing Tips
Many students feel that if they write something in complicated
language, they sound more intelligent. This results in
awful
sentences such as, “
A significant frequency of DNA is made of gene.” “Genes are

8. made of DNA.” makes a lot more sense!
Also, the term “significant” is only used with an accompanying
statistical test. See below for more helpful writing tips:
1) The phrasing,
",
so..." is conversational English, and not appropriate for written
English.
2) The word "very" has little meaning. Use a stronger
adjective. Four letter V-WORD.
3) Use the passive voice, not "We define
recombination frequency
as..." Instead use: "
Recombination frequency is
defined as..."
4) Separate different sections into paragraphs so the overall
organization is clear to the reader.
5) If you want to use "it" or "they" in a sentence, be certain that
the subject referred to is clear.
6) Omit needless words. Go through each sentence to reduce
wordiness.
7)” it’s” = it is; “its” is the possessive.
8) Do not keep using the word “it” in your complex sentences.
Re-word the sentence so the subject is clear.
9) Avoid meaningless sentences such as “
Chromosomes are
interesting molecule
s
that
are
found in
Drosophila
.” Think of a real point you want to make, and use meaningful

9. language.
10) Avoid contractions; don’t use them! I cannot emphasize
this enough; they’re too informal.
11) Semicolons separate two independent clauses; independent
clauses can serve as their own sentence.
12) A colon separates one independent and one dependent
clause: as in this sentence.
13) The possessive is rarely used in scientific writing and
comes off as awkward and unprofessional. Do not write, “The
gene’s
location is not known
.” Instead, write, “The
location of the gene is not known
.”