Assignments for majors
To begin an experiment, you must first design the phenotypes for the flies that will be mated. In addition to wild-type flies, 29 different mutations of the common fruit fly, Drosophila melanogaster, are included in FlyLab. The 29 mutations are actual known mutations in Drosophila. These mutations create phenotypic changes in bristle shape, body color, antennae shape, eye color, eye shape, wing size, wing shape, wing vein structure, and wing angle. For the purposes of the simulation, genetic inheritance in FlyLab follows Mendelian principles of complete dominance. Examples of incomplete dominance are not demonstrated with this simulation. A table of the mutant phenotypes available in FlyLab can be viewed by clicking on the Genetic Abbreviations tab which appears at the top of the FlyLab homepage. When you select a particular phenotype, you are not provided with any information about the dominance or recessiveness of each mutation. FlyLab will select a fly that is homozygous for the particular mutation that you choose, unless a mutation is lethal in the homozygous condition in which case the fly chosen will be heterozygous. Two of your challenges will be to determine the zygosity of each fly in your cross and to determine the effects of each allele by analyzing the offspring from your crosses.
One advantage of FlyLab is that you will have the opportunity to study inheritance in large numbers of offspring. FlyLab will also introduce random experimental deviation to the data as would occur in an actual experiment! As a result, the statistical analysis that you will apply to your data when performing chi-square analysis will provide you with a very accurate and realistic analysis of your data to confirm or refute your hypotheses.
For your ease in completing each assignment, the background text relevant to the experiment that you will perform is italicized, instructions for each assignment are indicated by plain text, and questions or activities that you will be asked to provide answers for are indicated by bold text.Assignment 1:
What effect, if any, does this have on the results produced and your ability to perform chi-square analysis on these data? If any of your crosses do not follow an expected pattern of inheritance, provide possible reasons to account for your results.
Develop a hypothesis to predict the results of this cross and describe each phenotype that you would expect to see in both the F1 and F2 generations of this cross.
Analyze the results of each cross by Chi-square analysis and save your data to your lab notes as previously described in the assignments for a monohybrid cross.
Describe the phenotypes that you observed in both the F1 and F2 generations of this cross. How does the observed phenotypic ratio for the F2 generation compare with your predicted phenotypic ratio? Explain your answer.
Develop a hypothesis to predict the results of this cross and describe each phenotype that you would expect to see in the F2 generation of this cross. Perform your cross and evaluate your hypothesis by Chi-square analysis. What was the trihybrid phenotypic ratio produced for the F2 generation?
A testcross is a valuable way to use a genetic cross to determine the genotype of an organism that shows a dominant phenotype but unknown genotype. For instance, using Mendel's peas, a pea plant with purple flowers as the dominant phenotype could have either a homozygous or a heterozygous genotype. With a testcross, the organism with an unknown genotype for a dominant phenotype is crossed with an organism that is homozygous recessive for the same trait. In the animal- and plant-breeding industries, testcrosses are one way in which the unknown genotype of an organism with a dominant trait can be determined. Perform the following experiment to help you understand how a testcross can be used to determine the genotype of an organism.
To determine the genotype of an F1 wild-type female fly, design a male fly with brown eye color and ebony body color, then cross this fly with an F1 wild-type female fly. Examine the results of this cross and save the results to your lab notes.
What was the phenotypic ratio for the offspring resulting from this testcross? Based on this phenotypic ratio, determine whether the F1 wild-type female male was double homozygous or double heterozygous for the eye color and body color alleles. Explain your answer. If your answer was double homozygous, describe an expected phenotypic ratio for the offspring produced from a testcross with a double heterozygous fly. If your answer was double heterozygous, describe an expected phenotypic ratio for the offspring produced from a testcross with a homozygous fly.
Five of the mutations in FlyLab are lethal when homozygous. When you select a lethal mutation from the Design view, the fly is made heterozygous for the mutant allele. If you select two lethal mutations that are on the same chromosome (same linkage group, or the "cis" arrangement), then the mutant alleles will be placed on different homologous chromosomes (the "trans" arrangement). Crosses involving lethal mutations will not show a deficit in the number of offspring. FlyLab removes the lethal genotypes from among the offspring and "rescales" the probabilities among the surviving genotypes. Hence, the total number of offspring will be the same as for crosses involving only nonlethal mutations. Perform the following crosses to demonstrate how Mendelian ratios can be modified by lethal mutations.
What phenotypic ratio did you observe in the F1 generation? What were the phenotypes? Perform an F1 cross between two flies with the aristapedia phenotype. What phenotypic ratio did you observe in the F2 generation? How do these ratios and phenotypes explain that the aristapedia mutation functions as a lethal mutation?
To convince yourself that the aristapedia allele is lethal in a homozygote compared with a heterozygote, perform a cross between a wild-type fly and a fly with the aristapedia mutation.
What results did you obtain with this cross?
Develop a hypothesis to predict the phenotypic ratio for the F1 generation. Mate these flies. What phenotypic ratio did you observe in the F1 generation?
Test your hypothesis by Chi-square analysis. Repeat this procedure for an F1 cross between two flies that express the curly wing and stubble bristle phenotypes.
Are the phenotypic ratios that you observed in the F2 generation consistent with what you would expect for a lethal mutation? Why or why not? Explain your answers.
The genetic phenomenon called epistasis occurs when the expression of one gene depends on or modifies the expression of another gene. In some cases of epistasis, one gene may completely mask or alter the expression of another gene. Perform the following crosses to study examples of epistasis in Drosophila.
What did you observe in the F1 generation? Note: It may be helpful to click up and down in this display box to closely compare the phenotypes of the F1 and P generations.
Was this what you expected? Why or why not? Once you have produced an F1 generation, mate F1 flies to generate an F2 generation.
Study the results of your F2 generation, then answer the following questions.
Which mutation is epistatic? Is the vestigial mutation dominant or recessive? Determine the phenotypic ratio that appeared in the dihybrid F2 generation, and use chi-square analysis to accept or reject this ratio.
Which mutation is epistatic? Is the apterous wing mutation dominant or recessive?
For many of the mutations that can be studied using FlyLab, it does not matter which parent carries a mutated allele because these mutations are located on autosomes. Reciprocal crosses produce identical results. When alleles are located on sex chromosomes, however, differences in the sex of the fly carrying a particular allele produce very different results in the phenotypic ratios of the offspring. Sex determination in Drosophila follows an X-Y chromosomal system that is similar to sex determination in humans. Female flies are XX and males are XY. Design and perform the following crosses to examine the inheritance of sex linked alleles in Drosophila.
What phenotypes and ratios did you observe in the F1 generation?
Mate two F1 flies and observe the results of the F2 generation.
Based on what you know about Mendelian genetics, did the F2 generation demonstrate the phenotypic ratio that you expected? If not, what phenotypic ratio was obtained with this cross?
Is there a sex and phenotype combination that is absent or underrepresented? If so, which one? What does this result tell you about the sex chromosome location of the white eye allele?
Mendel's law of independent assortment applies to unlinked alleles, but linked genes--genes on the same chromosome -- do not assort independently. Yet linked genes are not always inherited together because of crossing over. Crossing over, or homologous recombination, occurs during prophase of meiosis I when segments of DNA are exchanged between homologous chromosomes. Homologous recombination can produce new and different combinations of alleles in offspring. Offspring with different combinations of phenotypes compared with their parents are called recombinants. The frequency of appearance of recombinants in offspring is known as recombination frequency. Recombination frequency represents the frequency of a crossing--over event between the loci for linked alleles. If two alleles for two different traits are located at different positions on the same chromosome (heterozygous loci) and these alleles are far apart on the chromosome, then the probability of a chance exchange, or recombination, of DNA between the two loci is high. Conversely, loci that are closely spaced typically demonstrate a low probability of recombination. Recombination frequencies can be used to develop gene maps, where the relative positions of loci along a chromosome can be established by studying the number of recombinant offspring. For example, if a dihybrid cross for two linked genes yields 15% recombinant offspring, this means that 15% of the offspring were produced by crossing over between the loci for these two genes. A genetic map is displayed as the linear arrangement of genes on a chromosome. Loci are arranged on a map according to map units called centimorgans. One centimorgan is equal to a 1% recombination frequency. In this case, the two loci are separated by approximately 15 centimorgans. In Drosophila, unlike most organisms, it is important to realize that crossing over occurs during gamete formation in female flies only. Because crossing over does not occur in male flies, recombination frequencies will differ when comparing female flies with male flies. Perform the following experiments to help you understand how recombination frequencies can be used to develop genetic maps. In the future, you will have the opportunity to study genetic mapping of chromosomes in more detail using PedigreeLab.
Draw a map that shows the map distance (in map units or centimorgans) between the locus for the shaven bristle allele and the locus for the eyeless allele.
What is the phenotype of these flies? What does this tell you about the position of the purple eye allele compared with the black body and vestigial wing alleles? Sketch a genetic map indicating the relative loci for each of these three alleles, and indicate the approximate map distance between each locus.
Work in pairs to complete the following assignment. Each pair of students should randomly design at least two separate dihybrid crosses of flies with mutations for two different characters (ideally choose mutations that you have not looked at in previous assignments) and perform matings of these flies. Before designing your flies, refer to the Genetic Abbreviations chart in FlyLab for a description of each mutated phenotype. Or view the different mutations available by selecting a fly, clicking on each of the different phenotypes, and viewing each mutated phenotype until you select one that you would like to follow. Once you have mated these flies, follow offspring to the F2 generation.