Genetics Exercises

Complete the gene exercise. Refer to attachment.

Exercise I
We are going to begin by looking at some basic mating patterns, using a couple of eye color mutations in fruit flies.

The normal fruit fly has brick red eyes. This is called the "wild type," because the vast majority of wild fruit flies have this color eyes. One unusual eye color is "brown," which is caused by a mutation in a particular gene.

1. Our first matings will between two homozygous flies, one with brown eyes, the other with the wild type. We'll try both versions of this mating ? female wild x male brown, and female brown x male wild. These two crosses are reciprocals of each other. Click on the links below to see the results of these two matings.

Female Wild Type x Male Brown Female Brown x Male Wild Type

Offspring: Offspring:
Males Females Males Females
Wild 5061 4933 Wild 5000 4928
Brown 0 0 Brown 0 0

2 Record the results for these two matings on your data sheet, including the numbers and the ratios. These are the F1 flies in your first experiment. NOTE: Calculate approximate phenotypic ratios by dividing all of the results numbers by the smallest of the results numbers. See this example.

3 From the results of this cross, which is dominant, wild type or brown eyes? Record your answer on the data sheet.
4 Assign symbols to the two alleles of this brown eyed gene. Record your choices on the data sheet. Remember that there are a couple of rules about assigning symbols for alleles.

5. What are the most probable genotypes of the F1 flies? Record your answer on the data sheet.

6. You are now going to create an F2 generation. For the next stage, we are going to mate these F1 flies to each other. This mating (only one gene followed, both parents heterozygous) is called a monohybrid cross. Before you go to the next step, predict the results of this mating by working the genetics problem it represents. What phenotypic ratio do you predict among the F2? Be sure to fill in your work for this mating on the data sheet.

7. Mate these flies by clicking on the link below. Record the results of this mating, including the phenotypes, numbers and ratios, on the data sheet.

8. Answer the following questions about this series of matings:
a. Did your F2 produce the results you'd predicted?
b. Is this gene sex linked or autosomal?
c. What led you to decide what the answer to #2 was?

Exercise II

On to our second eye color mutation in fruit flies. This time we are going to look at a different gene that affects fruit fly eye color. Again, the wild type will be that brick red color ? that's the wild type for all of the eye color genes in fruit flies (and there are a lot). The mutant allele of this gene produces flies with white eyes. Before you do any matings, look back at exercise one, and remind yourself of the way things went with the brown eye gene. We are going to do the same matings with this gene.

1. First, make some predictions. We're going to start by making a couple of F1 generations. Record your predictions on the data sheet.
a. What phenotypic ratio do you predict you'll get if you make an F1 by mating a wild type female with a white eyed male?

b. What phenotypic ratio do you predict you'll get if you make an F1 by mating a white eyed female with a wild type male?

2 Now, click on the following links to see what actually happens in these matings. Record the results on the data sheet.

3 Did you get what you predicted? Can you explain this?

4 We are going to make our F2 with the offspring of the wild type female and the white eyed male. What are their genotypes? Record them on your data sheet. What color eyes do these two flies have? Record that, too. Then click on the link to mate the two flies.

5 Record the results of this mating on your data sheet.

6 Compare the results of this mating to the results of the monohybrid cross in exercise I, and answer the following questions.
a. Are the phenotypic results the same?

b. Look carefully at the two sets of results. In what very significant way are they different?

Exercise III

In this exercise, we are going to consider several different body color mutations in fruit flies. Some of these are autosomal, some are X linked. Your task is to look at the results of some matings and determine which are which. You might want to go back and review the F1 results you saw in Exercises I and II.

Wild type flies have a striped black and yellow body. We are going to be looking at five different body color mutations, all caused by different genes. The mutations we are looking at will be black body, sable body, ebony body, tan body and yellow body. You will be able to see the results of reciprocal crosses for each of these five genes. In all crosses, you may assume that the parents are homozygous.

1. Before you make any matings, answer the following questions on your data sheet:
a. Which kind of mating do you expect to be most useful in deciding which of these genes are sex linked, wild female x mutant male, or mutant female x wild male?

b. Why? What results will indicate X linkage, and what will indicate autosomal?

2 Now click on any of the following links you think will help you figure this out:

3. Which of these genes is X linked, and which autosomal? Enter your answers on the data sheet.

Exercise IV

This time you will be following two genes at the same time. You're going to use the ebony body gene, and the purple eyes gene. Sounds like a lovely outfit ;^) This should demonstrate independent assortment. We will mate an ebony body, purple eyed female to a male who is homozygous for both of these genes.

1. Before you mate these flies, answer the following question on your data sheet: Assuming that each of these traits is recessive and autosomal, predict what the F1 flies will look like.

2 Click on the link below to mate these flies. Was your prediction correct?

3 Now you want to make an F2 by mating the F1 females to the F1 males. Before you do this, consider the following. Remember that you are assuming that these genes are autosomal

a. For the moment, ignore the eye color gene and consider only the body color gene. Refer to Exercise I if you need to. If we mate these two flies, what fraction of them do you predict should have an ebony body?

b. Now ignore the body color gene and consider only the eye color gene. What fraction of these offspring should have purple eyes?

c Consider the following statement for a moment: If these two genes behave independently of each other, then we expect _______ (fill in your answer to question 1) of the purple eyed offspring to have ebony bodies. If that's true, then what fraction of the total bunch of F2 offspring should have both ebony bodies and purple eyes? Enter this answer on your data sheet. (Here's some help with these statistics.)

c. Use the reasoning in #3 to figure out the fraction of the offspring which should be wild type for both traits, the fraction which should have ebony bodies but wild type eyes, and the fraction that should have wild type bodies but purple eyes. Enter these predictions on your data sheet.

4 Finally you can mate your flies by clicking on this link. Enter the results on your data sheet.

5 Was your prediction reasonably correct?

6 So what can you conclude from this exercise? (Hint: Go back and review the assumptions you made in #3 above.)

Exercise V

This is an extra credit thought and reasoning exercise. You will need to think about genes and chromosomes, and about the process of meiosis to figure out the answer.

By clicking on the links below, repeat the same process you followed in Exercise IV. This time, our Parental flies will be a sepia eye, ebony body female and a homozygous wild type male

You should discover that you don't get the same F2 results for this as you got for Exercise IV. Can you reason out why?