Probability Exercise

1994 Woodrow Wilson Biology Institute

Introduction:

In Drosophila melanogaster (the wonderful fruit fly you generally find hanging around the farmers' markets) normal wings (W) are dominant over vestigial wings (w); gray body color (G) is dominant over ebony color (g); normal antennae (A) is dominant over antennapedia (a) and the sex-linked trait of red eyes, called the wild type, (R) is dominant over white eyes (r).

Activity:

1. Bugsy, a male fruit fly, who is homozygous dominant for body color marries Daisy, a female fruit fly, who is homozygous recessive for body color.

1. Write the genotypes of Bugsy and Daisy for body color. Bugsy and Daisy will be the P generation.
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2. What are the two alleles that Bugsy can donate for body color?
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What are the two alleles that Daisy can donate for body color?
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3. Draw a Punnett square and show the possible genotype(s) of the F1 generation produced by Bugsy and Daisy.
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4. What is the percentage of genotype(s) found in the F1 generation?
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5. What are the alleles for body color that can be donated by any member of the F1 generation? ____________________________________________________________

6. Now cross two individuals from the F1 generation (a monohybrid cross). Show this cross. The offspring from this cross is the F2 generation!

7. What are the genotypic ratios for body color in the F2 generation?
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8. What are the phenotypic ratios for body color in the F2 generation?
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2. Now let us carry this one step further and work with two traits. Dino, who is a good friend of Bugsy, is homozygous recessive for body color and homozygous dominant for antennae. He meets, falls in love and marries Daisy's sister Lulu. (Yes, true love can happen in fruit flies!) Lulu is homozygous dominant for body color and homozygous recessive for antennae.

1. What are the genotypes of the P generation (Dino and Lulu)? Be sure to include both traits!
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2. Draw the Punnett square and show the cross between Dino and Lulu. (Remember that their offspring will be the F1 generation.)

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3. What are the percentages of the genotype(s) found in the F1 generation?
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4. List the 4 possible allelic gamete combinations that could be produced by two members of the F1 generation.

5. Now cross two individuals from this F1 generation. Show the cross in a Punnett square.
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6. Using crayons or coloring pencils, shade in those squares on the Punnett square that are phenotypically alike.

7. What is the ratio of phenotypes we can expect from this dihybrid cross?
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8. What was the point of this exercise?

1. To learn to use coloring pencils and stay within the lines
2. To waste a class period
3. To show all the possible phenotypes offspring could have
4. To keep the teacher happy

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