biology

Fruit Fly lab:

Design of the Experiment

How to Tell the Males from the Females The first thing you must do in this laboratory is learn to distinguish male fruit flies from female fruit flies.

As illustrated in the figures above, female flies generally have a long, pointed, lightly striped abdomen. The male abdomen, by contrast, is blunt and darker. Very young flies may not show this clear difference, so for more certain sex identification you should look for sex combs, dark bristles found on the inner surface of the forelimbs. Only males have sex combs.

Some common mutations you may see in the F1 generation:

Week One You have a vial containing both male and female flies. This is the parental generation. You are starting with 2 red eyed parents. You cannot tell from the phenotype whether these are homozygous or heterozygous for red eyes. The results of your cross will allow you to make this determination. This is your P generation.

The flies that hatched from the initial vial are the F1 generation. When looking at the results tables, be sure to observe whether there is any relationship between sex and the traits in the offspring.

Data table 1: Results of the P generation cross.

Case 1

Set up a new vial and transfer some of the F1 flies to it.

Data Table 1:

Data table 2: Results of the F1 cross

Case 2

The original P generation and the F1 results look the same as in Case 1. Data table 3 shows the F2 results in the Case 2 cross

Data table 3:

Chi-Square Analysis of Data

Look at the tables from Case 1. From the data presented, you can deduce that the F1 cross was between individuals heterozygous for eye color:

+sex +se (+ = red; se = sepia ). From this conclusion, you could write the following hypothesis: "If the parents are heterozygous for eye color, there will be a 3:1 ratio of red eyes to sepia eyes in the offspring." Do your results support this hypothesis?

The actual results of an experiment are unlikely to match the expected results precisely. But how great a variance is significant? One way to decide is to use the chi-square (x2) test. This analytical tool tests the validity of a null hypothesis, which states that there is no statistically significant difference between the observed results of your experiment and the expected results. When there is little difference between the observed results and the expected results, you obtain a very low chi-square value; your hypothesis is supported.

Next we'll see how to calculate and interpret chi-square.

How to Calculate the Expected Value

In a cross between two heterozygous individuals, the offspring would be expected to show a 3 : 1 ratio. For example, in Case 1, three-fourths of the individuals would have red (wild-type) eyes, and one-fourth would have sepia eyes.

If there are 44 offspring, how many are expected to have red eyes?

We expect three-fourths to have red eyes.

If there are 44 offspring, how many are expected to have sepia eyes?

Now you are ready to calculate chi-square.

Calculating Chi-Square

The formula for chi-square is: X2 = the sum of

where: o = observed number of individuals e = expected number of individuals

Using the data from Case 1, complete this table:

Phenotype	Observed No. (o)	Expected No. (e)	(o-e)	(o-e)2	( o - e )2 e
Red eyes
Sepia eyes

2 (to the nearest ten-thousandth)

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Questions:

1.	Why is it important to remove the adults in the parental generation? Your Answer:
2.	What generation will their offspring be? Your Answer:
3.	Based on the data obtained, is the cross in Case 1 monohybrid or dihybrid? Explain. Your Answer:
4.	Is the cross in Case 1 sex-linked or autosomal? Explain. Your Answer:
5.	Based on the data obtained, is the most likely mode of inheritance in Case 2 autosomal or sex-linked? Explain. Your Answer:
6.	From the data presented, what is the genotype of the parental (before the F1 generation; not shown here) generation? Your Answer:
7.	Determine the degrees of freedom. This is the number of categories (red eyes or sepia eyes) minus one. For the data in Case 1, what is the number of degrees of freedom? Your Answer:
8.	Find the probability (p) value for 1 degree of freedom in the 0.05 row. Compare this with the chi-square value you calculated in your Lab Notebook. What can you say about the null hypothesis? Your Answer:

What Can Fruit Flies Reveal About Inheritance? Lab

Complete the Mastering Biology Lab: What Can Fruit Flies Reveal About Inheritance? Located in the Mastering Biology Interactive Media link. Complete the "What Can Fruit Flies Reveal About Inheritance? Lab" worksheet by taking notes from the investigation and answering the questions associated with the investigation. Click the Assignment Files tab to submit your assignment.

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