Getting to Know FlyLab: Performing Monohybrid, Dihybrid, and Trihybrid Crosses 1.
To begin a cross, you must first select the phenotypes of the flies that you want to mate. Follow the directions below to create a monohybrid cross between a wild-type female fly and a male fly with sepia eyes. a.
To design a wild-type female fly, click on the Design button below the gray image of the female fly. Click on the button for the Eye Color trait on the left side of the Design view. The small button next to the words "Wild Type" should already be selected (bolded). To choose this phenotype, click the Select button below the image of the fly at the bottom of the design screen. Remember that this fly represents a true-breeding parent that is homozygous for wild type alleles. The selected female fly now appears on the screen with a "+" symbol indicating the wild-type phenotype. b.
To design a male fly with sepia eyes, click on the Design button below the gray image of the male fly. Click on the button for the Eye Color trait on the left side of the Design view. Click on the small button next to the word "Sepia." Note how eye color in this fly compares with the wild-type eye color. Choose this fly by clicking on the Select button below the image of the fly at the bottom of the Design screen. The male fly now appears on the screen with the abbreviation "SE" indicating the sepia eye mutation. This fly is homozygous for the sepia eye allele. These two flies represent the parental generation (P generation) for your cross. c.
Based on what you know about the principles of Mendelian genetics, predict the phenotypic ratio that you would expect to see for the F1 offspring of this cross and describe the phenotype of each fly. I believe that all of the F1 offspring will demonstrate the same eye color because according to Mendelian genetics, when both members of the P generation are homozygous for a characteristic one of the 2 characteristics will be dominant in the F1 offspring. d.
To select the number of offspring to create by this mating, click on the popup menu on the left side of the screen and select 10,000 flies. To mate the two flies, click on the Mate button between the two flies. Note the fly images that appear in the box at the bottom of the screen. Scroll up to see the parent flies and down to see the wild type offspring. These offspring are the F1 generation. Are the phenotypes of the F1 offspring what you would have predicted for this cross? Why or why not? The results for the F1 offspring did turn out to be what I predicted. All of the F1 offspring inherited the dominant wild type eye color. Note: The actual number of F1 offspring created by FlyLab does not exactly equal the 10,000 offspring that you selected. This difference represents the experimental error introduced by FlyLab. e.
To save the results of this cross to your lab notes, click on the Results Summary button on the lower left side of the screen. A panel will appear with a summary of the results for this cross. Note the number of offspring, proportion of each phenotype and observed ratios for each observed phenotype. Click the Add to Lab Notes button at the bottom of the panel. Click the OK button to close this panel. To comment on these results in your lab notes, click on the Lab Notes button and move the cursor to the space above the dashed line and type a comment such as, "These are the results of the F1 generation for my first monohybrid cross." Click the Close button to close this panel and return to the Mate screen. f.
To set up a cross between two F1 offspring to produce an F2 generation, be sure that you are looking at the two wild-type offspring flies in the box at the bottom of the screen. If not, scroll to the bottom of this box until the word "Offspring" appears in the center of the box. Click the Select button below the female wild-type fly image, then click the Select button below the male wild-type fly image. Note that the...
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