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Laboratory Experiment 7: Genetics of Organisms for AP Biology

By — McGraw-Hill Professional
Updated on Oct 24, 2011

This experiment deals with material from heredity. This is your chance to be Mendel or Morgan for a few weeks. You will be breeding fruit flies to apply the principles of genetics and heredity you learned during the course of the year. The basic goal of this experiment is to teach you how to determine what kind of inheritance patterns certain genes are displaying.

For a quick review on heredity, refer to the following concepts:

The life cycle of a fruit fly (Drosophila) is in the ball park of about 12–14 days in length. The basic life plan is as follows:

Eggs → larva → pupae → adults

Egg stage: hatch to become larva after about 24 hours.

Larva stage: 4–7 days in duration, major growth period for the fruit fly.

Pupal stage: lasts about 4 days; wings, legs, and eyes become visible.

Adult stage: live for about 30 days.

Here is an interesting fruit fly fact you can pull out at a party to impress everyone around you (or at least use to grind the party conversation to a halt as everyone stares at you wondering what planet you are from). Once female fruit flies have mated, they store sperm that they can use for fertilization for quite a long time. This means that if you are going to run a Morgan-like experiment, you must use virgins because otherwise you cannot be sure who the daddy is.

In this experiment different students study different inheritance modes—monohybrid, dihybrid, and sex-linked. Essentially, each group of students takes a vial of experimental flies, which will be the parental generations. That vial contains eggs and larva that represent the F1 generation. During the first portion of the experiment, the phenotypes for the characteristics that you are attempting to study are recorded—which flies had which phenotypes. Next the phenotypes of the F1 flies are noted and recorded. When the F2 generation has reached adulthood, their phenotypes are recorded as well. As is often the case, the more the merrier, since experimental results tend to pull more weight when the sample size is larger.

Results from Fruit Fly Matings

After several weeks pass and all the phenotypic data have been collected, it is now time to analyze the data. Heredity, some genetic ratios were mentioned; I will rewrite them here for you:

3 : 1               Monohybrid cross

9 : 3 : 3 : 1       Dihybrid cross

9 : 4 : 3           Epistasis

1 : 1               Linked genes

4 : 4 : 1 : 1       Linked genes, with some crossover.

In this experiment, you were taught how to do chi-square (Χ2) analysis to evaluate the results of your genetic crosses. Occasionally you will be asked to perform this type of analysis on the AP Biology exam, so be sure to review this technique. It is more realistic that you simply know what a chi-square test is used for: to determine if your results conform to the expected Mendelian frequencies. For example, if you get 96 : 31 : 45 : 2 in your F2 generation, a chi-square test will help you decide if your experiment was a dihybrid cross. If your observed frequencies do not match your expected frequencies, perhaps some nonrandom mating or even crossover is occurring.

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