Drosophila Lab—Patterns of Heredity

Objective: Learn and apply the principles of Mendelian inheritance by experimentation with the fruit fly Drosophila melanogaster. Make hypotheses for 2 Experiments: 1. Monohybrid crosses, 2. Sex– linked crosses, and test hypotheses by selecting fruit flies with different mutations, mating them, recording observations, and analyzing the phenotypic ratios of the offspring.

Website: https://cgslab.com/genetics/index.html
Choose “Create your own Drosophila population.”
You will see a diagram of the 4 Drosophila chromosomes. REMEMBER – Chromosome 1 is the Sex Chromosome X! That is where you will find the sex–linked traits. Traits starting with small letters are recessive traits (need 2 copies). Traits starting with capital letters are dominant traits (need only 1 copy to express the condition). “WILD TYPE” means “normal” – no condition.

Experiment 1: MONOHYBRID CROSS (One Trait for Body Color)

1. Choose one trait “e” for Ebony Body Color that is located on Chromosome 3. You can hover your cursor over the “e” to see what it is and then click on it. This is an autosomal (not sex–linked) mutation. Then click CONTINUE.

2. You will get 100 flies – some have Wild Type (normal body color) phenotypes and some have Ebony phenotypes. Some are male and some are female. Hover your cursor over the flies to see what they are. Drag a female Wild Type and a male Ebony to the blue Crossing Vial so they can mate (click on “Cross Flies” to create offspring in Vial 1).

3. Before looking at the analysis, what do you think happened? Based on the mating you performed, fill out your hypothesized PUNNETT SQUARE for Generation:___ F1 ____ Trait:___ Ebony (e) ________

4. What is your hypothesis of the ratio of offspring phenotypes?

5. Click on “Analysis” to find out what types of offspring you have in Vial 1. Because this is NOT a sex– linked trait, the sex of the offspring doesn’t matter, so just combine the sexes and use the total number of flies for each phenotype.

6. Click on “Stats” and look at the trait of “Body Color” for “All” offspring (both male & female). The Observed number of each phenotype is the actual number of offspring resulting from the mating you performed. The Expected number of each phenotype is how many offspring you would theoretically expect in a perfect world from just doing the Punnett square. IN BOTH COLUMNS, this will add up to your total number of offspring. For example, if you had 84 total offspring, and you expected (based on your Punnett square) 50% of one phenotype and 50% of the other, that would be 42 flies of one phenotype and 42 flies of the other. For another example, if you had 84 total offspring and you expected 75% of one phenotype and 25% of the other, that would be 84*.75= 63 of one phenotype and 84*.25=21 of the other (which still adds to 84). Enter your expected numbers in the simulation program and click on “CALCULATE.” If you get the “Divide by 0 error” because your expected and observed numbers are the same, that is the same as getting a very large p–value (greater than 0.05) and the ratio of offspring phenotypes is the same as what you expected. Just write “very large” in the chart for the p–value.

  • Male Parent
  • Alleles
  • Female
  • Parent
The computer will do a Chi Square statistical analysis and give you a “level of significance” called a p–value. You can ignore the Chi Squared value and the Degrees of Freedom. If the p–value is < 0.05, then your hypothesis is rejected and your conclusion is that the ratio of offspring phenotypes is different than the ratio you expected. If the p–value is > 0.05, then your hypothesis is NOT rejected and your conclusion is that the ratio of offspring phenotypes is the same as what you expected.

7. Fill in all 10 blank squares of the chart

8. What was the genotype of YOUR original Female Wild Type parent? Why?

9. What other genotype could a fly with a Wild TYPE phenotype possibly be?

10. Now mate the offspring (F1) from that cross together (creates F2 generation). From your Vial 1 go back to the “Organisms” tab and drag a Wild Type female and a Wild Type male into the Crossing Vial to mate and produce offspring in Vial 2.

11. Based on the mating you performed, fill out your hypothesized PUNNETT SQUARE for Generation:___ F2 ______Trait(s):___ Ebony (e) __________________________

12. What is your hypothesis of the ratio of offspring phenotypes?
Phenotype (description) # Observed # Expected
Total #
p–value
Conclusion
Male Parent
Alleles
Female
Parent

13. Fill in all 10 blank squares of the chart from the “Analysis” and “Stats” tabs from Vial 2:

14. What was the genotype of YOUR two original Wild Type parents? Why (based on your results)?

15. Now you can “Destroy” your vials of flies and EXIT back to the start to set up Experiment 2.
Experiment 2: SEX–LINKED TRAITS
A reciprocal cross is two matings: a Wild Type female x mutant male followed by a mutant female x Wild Type male. (Obviously, you study the same trait here). Choose the sex–linked trait “w” for White Eyes found on the sex chromosome I–X. Do both crosses and fill out your hypothesized Punnett Squares and Chi Square analysis charts for both: 1st mating: Wild Type female x mutant White–eyed male to produce offspring in VIAL 1:

16. Based on the mating you performed, fill out your hypothesized PUNNETT SQUARE for Generation:___ F1 ______Trait(s):___ White Eyes (w) ____________________ REMEMBER TO USE X and Y chromosomes with the allele as a superscript!

17. What is your hypothesis of the ratio of offspring phenotypes?
Phenotype (description) # Observed # Expected
Total #
p–value
Conclusion
Male Parent
Alleles
Female
Parent

18. This time under STATS, choose to analyze BOTH Eye Color and Sex. Your total numbers of flies will be separate for males and females. Fill out the 18 blank spaces in the chart:

19. What was the genotype of YOUR original Female Wild Type parent? Why?

20. What other genotype could a female fly with a Wild TYPE phenotype possibly be? 2nd mating: (Go back to Wild Population “Organisms”) and cross a Mutant White–eyed female x Wild Type male to produce offspring in VIAL 2:

21. Based on the mating you performed, fill out your hypothesized PUNNETT SQUARE for Generation:___ F1 ______Trait(s):___ White Eyes (w) ____________________ REMEMBER TO USE X and Y chromosomes with the allele as a superscript!

22. What is your hypothesis of the ratio of offspring phenotypes?
Phenotype (description) # Observed # Expected
Male
Male
Total #
Female
Female
Total #
p–value
Conclusion
Male Parent
Alleles
Female
Parent

23. Again, choose to analyze BOTH Eye Color and Sex. Your total numbers of flies will be separate for males and females. Fill out the 18 blank spaces in the chart:

24. What are the genotypes of YOUR original two parent flies?

25. Are there any other possible genotypes your parent flies could be?

26. Do you get different results from the two different reciprocal matings when the trait is sex–linked?
Phenotype (description) # Observed # Expected
Male
Male
Total #
Female
Female
Total #
p–value
Conclusion