Step 1: Hypothesize

Fruit flies can have many eye colors that are different from the normal (or "wildtype") red color of most flies. One of those eye colors is white, which is an eye absent of pigment. Scientists are not 100% sure how this trait is inherited, although several experiments have suggested that it is sex-linked recessive.

White Eyed Female Wild-Type (Red Eyed) Male

In order to test the inheritance of the white-eyed trait for ourselves, we will be crossing white-eyed females with wildtype males. In order to best come up with your hypothesis, answer the questions below. Please note that although here a letter has been used to represent the wildtype genotype, in the research community wildtype flies are represented by "+" symbols.

What Results Should You Expect?

1. Which of the following genotypes shows a sex-linked recessive trait?
Rr.
rr.
XrXr.
XRY.

2. What should we predict the genotype of a white eyed female to be?
XeXe.
XEXE.
XEXe.
XeY.

3. What should we predict the genotype of a wildtype male to be?
XEYE.
XeY.
XEYe.
XEY.

4. On a separate sheet of paper, show the cross of the white eyed female to the wildtype male using a Punnett square (shown below). What percentage of the offspring in the F1 (first generation) will be white eyed females?
50%.
0%.
100%.
75%.

5. According to your Punnett square, what percentage will be red eyed females?
50%.
75%.
100%.
0%.

6. What will the offspring ratio be?
3:1 red males to white females.
2:1 red males to white females.
1:1 red males to white females.
3:1 white females to red males.

7. On your Punnett square, you wrote letters above each space. What does each single letter represent?
A complete genotype.
A gamete that contains one allele of the gene from the parent.
A gamete that contains a gene from the parent.
A body cell that contains a gene from the parent.

8. Why must a fly have at least two recessive alleles in order to show the white eye color phenotype?
Because this is a recessive trait, the fly cannot have alleles that encode for normal eye color proteins. This would cause the fly to have normal eye pigmentation.
This is false; a fly that is heterozygous still has half the protein for white eye color and will therefore show the white eye color phenotype.
The big E is capital and therefore stronger than the little e, so if any big E is present it will cover up the little e.