Genetics of Organisms

NOTE: At your station, you should have four vials of flies. They will be labeled as male or female. They will also be labeled with the notation REF for red-eyed flies or WEF for white-eyed flies. The female vial should also be labeled virgin as these flies should be unmated for the purpose of this experiment.

Make sure you have around five flies in each vial. NOTE: You will perform crosses using these flies to determine if red or white eyes is dominant and if the trait is sex-linked. You should also have two fly anesthesia wands, a paint brush, magnifying glass, fly anesthetic, some empty fly vials, and a sheet of white paper.

Draw a line in the middle of the paper to make a male side and a female side.

Dip the fly anesthesia wand into the solution and insert it into the red-eyed female vial until the flies stop moving.

Then, dip the same wand into the white-eyed male vial until these flies are also anesthetized.

Tap the flies from the two vials onto the sheet - Females on one side and males on the other.

Check to make sure that all the flies from the female vial are female and likewise, confirm that the male flies are male. NOTE: Males are slightly smaller and have a darker solid pigment on the end of their abdomen. Females are larger and have stripes rather than a solid spot.

If there are any male flies in the female vial, obtain a new vial as these females likely have sperm from males from their own strain and are unusable.

Now, move the five red-eyed females and five white-eyed males into a new vial using the paintbrush.

Label this vial to indicate that this is a cross between red-eyed females and white-eyed males. HYPOTHESES: Use Punnett squares to formulate hypotheses for this experiment. Consider what you will see in a few days if the white allele is dominant and there is no effect of parental sex. What if red is dominant? Under the potential offspring genotype in each box, write the expected phenotype.

To set up the reverse cross, again dip the fly anesthesia wand into the solution and insert it this time into the white-eyed female vial and then the red-eyed male vial until all the flies have stopped moving.

Tap the flies onto a sheet of white paper and check to make sure that all the flies from the female vial are female and likewise with the male flies.

Now, move the five white-eyed females and five red-eyed males into a new vial using the paintbrush.

Label this vial to indicate that this is a cross between white-eyed females and red-eyed males.

Finally, place the two tubes containing the new crosses into a rack for incubation at room temperature. NOTE: After about three days, larvae should be moving around in the vial.

Remove the 10 adult flies and dispose of them into the morgue tube containing 70% ethanol.

Now, put the tubes back in the rack and let the larvae mature until they reach the adult stage in up to two weeks.

Once all of the larvae from these crosses have matured to adulthood, anesthetize the flies in one of the tubes using the wand.

Then, on a piece of white paper, sort the flies by sex and phenotype and then record these data.

Now, repeat this process and tally the sexes and phenotypes for the second cross (steps 1 – 18).

Finally, once you have counted all of your flies, dispose of them in the morgue vial.

To begin investigating your results, first compare the data you collected with the hypothesis you made using the Punnett squares.

Did your data match the simple Punnett square hypothesis where red or white is dominant and there is no effect of sex? Compare your data with other lab groups.

If your data does not match what your Punnett squares predicted, does this indicate that the gene may be sex-linked?

If the red gene were dominant and sex-linked, when we cross red-eyed females with white-eyed males, all of the offspring would have red eyes. Assuming all of the females were homozygous for the color allele, does this match your data for the first cross?

If white were dominant and the gene were sex-linked, we would see the exact opposite. So all of the flies would have white eyes. From your data, can you conclude from your first cross whether red or white is dominant?

When the reciprocal cross of white-eyed females and red-eyed males is performed, if the red phenotype is dominant, all of the females would be red and all of the males would be white. This would also confirm that the gene is specifically x-linked because white-eyed females could then only be produced in crosses with white-eyed males. Did your second cross produce any white-eyed females?

Finally, if the gene were x-linked and white was dominant, crossing white-eyed females with red-eyed males would result in all of the offspring having white eyes. Does this hypothesis fit with your second cross?

Look over your data and conclude which of the hypotheses best fits the patterns you found in your crosses. If your data does not fit with any of these, hypothesize why this might be the case.