FlyLab Experiments, Set II
Analyze the following traits using the
FlyLab. For each of these crosses you
will be given two traits to analyze at a time. Thus in
addition to determining the inheritance pattern of each
trait individually you need to explain the inheritance
patterns that occur when both mutations are involved in the
same cross (a dihybrid cross). In addition, you
should use the chi-square test to test any hypotheses that
you propose, if a chi-square test is possible for your
hypothesis. Don't forget to give phenotypic and genotypic
diagrams of all relevant crosses, with results and your
conclusions. You will work in pairs (assigned in class) and
just turn in one report per pair. There is a
scoring rubric for each of these
problems showing the characteristics expected in good
write-ups of the problems (it is different than the one for
FlyLab I so be sure to check it out). The four problems are
worth five points each and your results are due in your
discussion section, the week of Feburary 12th. I will
quickly walk you through setting up a dihybrid cross and
analyzing the results with a chi-square test, or you can
skip down to problem 1.
Sample walk -through: For a dihybrid cross we need two
different traits to follow. Let's try shaven bristles and
Step 1, designing the flies:
- Log in to the
- Click on the button that says "Start FlyLab" and wait
for the program to download to your computer (this may
take a couple of minutes). When finished downloading you
should see a a gray drawing of two fruit flies.
- As in VF 1, design a female fly with shaven bristles
and a male fly with brown eyes and then mate them.
- You should get an F1 generation that is
all wild-type (wt), suggesting that both of these traits
are recessive. Now cross the wt F1 to produce
the F2 generation.
- When the results come back you will see two wt
progeny and an arrow pointing down, below the statement
that there are "six fly images". If you look at the other
images you will see flys with shaven bristles, like the
parental female, flies with brown eyes, like the parental
male, and at the bottom, flies with both shaven bristles
and brown eyes. This is different than the result s we've
seen before because we have a new fly phenotype that
doesn't look like either of the parents or a wt.
- Click on "Analyze Results" to see the numbers for
these different types of progeny.
- There's lots of different numbers here but if you
look close you'll see that there is little difference
between the numbers for males and females so select the
radio button beside "Ignore sex".
- Now there should be just four sets of numbers, for
wild-type, shown as a +, shaven (SV), brown (BW) and
shaven and brown together, (SV,BW). The numbers will seem
a little strange, but there should be much more wt than
anything else and fewer SV, BW than the others.
Step 2, determing the expected ratios:
- If the traits are assorting independently (as
Mendel's pea traits did) than we would expect 3/4 of the
F2 to have wt bristles and 3/4 to have wt
eyes. If you add together the + flies and the BW flies
(which have wt eyes) the number should be about 3/4 of
the total (if you used the default this will be about
- Probability theory says that if 3/4 of the flies have
wt bristles and 3/4 of the flies have wt eyes and these
two traits are independent, then 3/4 x 3/4 = 9/16 of the
flies should be wt for both bristles and eyes.
- As 1/4 of the flies should have shaven bristles, then
we would expect 1/4 x 3/4 (wt eyes) = 3/16 SV
- The same type of calculation gives us 3/16 BW and 1/4
x 1/4 = 1/16 SV,BW. It may be hard to look at your
numbers and see if you really got those ratios,
especially as there can be a lot of variation in the
- To see if your numbers really match what we would
expect for independent recessive traits, we need to do a
statistical test called a chi-square test.
step 3, doing a Chi-Square test:
- Click on the tab labeled "Chi-Square Analysis" near
the top right corner of the results page.
- In the new window that appears we have the phenotypes
and numbers from before along with a column of empty
- First click on "Ignore Sex".
- To test our results we need to put in the expected
ratios in the empty fields. As we expected the numbers of
different fly types, as calculated above, to be in a
ratio of 9 + : 3 SV:3 BW: 1 SV, BW enter a 9 opposite the
+, a 3 opposite the SV, a 3 opposite the BW and a 1
opposite the SV, BW (you could also put in 9/16, 3/16,
3/16, and 1/16, but this is easier).
- Now click on "Test Hypothesis".
- You should get back a table with the Chi-Square Term
for each row and a total "Chi-Square Test Statistic" at
the bottom along with the degrees of freedom, which
should be 3, and the probability of getting a Chi-Square
Test Statistic as large as the one you got if your
hypothesis was true, the "Level of Significance".
- For scientific purposes, a level of significance
above .05 is acceptable and the program will give a
recommendation of "Do not reject your hypothesis".
- Most likely you have a chi-square score below 11 and
a recommendation to not reject your hypothesis.
- However, 1 time out of 20 the test will reject your
hypothesis, even if it is true.
- If this happens to you just ned to repeat the cross
and try the test again.
- If the traits are not independent or the traits are
not simple recessives with 3:1 ratios, then it will fail
this test again and you will need to come up with a new
hypothesis. This will almost certainly happen in some of
the following problems, so beware.
For each of the following problems you should determine
the inheritance pattern of each of the two traits, make a
prediction for what will happen in a dihybrid cross, and
test your prediction with a chi-square test. If the test
fails then you will need to come up with a new hypothesis to
explain your results, and , if possible, test that
hypothesis. You should do both an F2 cross (cross two F1
flies) and a test cross of a heterozygous F1. If you're
getting confused, you can always cross just one trait at a
time, as in the first assignment.
1. Investigate the radius incompletus wing and purple
eye traits (5 pts.)
2. Investigate the ebony body and curly wing traits (5
3. Investigate the eyeless and sepia eye traits. (5
4. Investigate the black body and apterous wing traits
Does it matter whether you start with a cross
between black and apterous or between black, apterous and
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Last Update: Friday, January 26, 2001