The X

The X-chromosome of the fruit fly Drosophila melanogaster can easily have a few of its genes mapped by crossing a wild-type fly with a multiple mutant. This exercise was also undertaken in July. Observing the phenotypes of the F₂ generation enables the students to determine the frequency of crossing over between the genes. The X-chromosome is the simplest to map because the males only possess one, so every allele present is visible in a male fly's phenotype.

The fly on the left is wild-type, with red eyes, straight bristles on the thorax, and wings longer than its body.

On the right is a multiple mutant with white eyes, bent bristles, and wings shorter than its body.

These features are easy to recognise.

With five groups working on it at once the distances between three of the fly genes can be determined in a single period.

The well-organised teams like this one have an operator who moves the fly into place under the microscope, an observer who calls out the features shown by the fly, and a recorder who fills in a table of the results. Several hundreds of flies need to be analysed to be able to calculate an accurate distance between the genes.

PREP

Use the information below to calculate the distances between the three genes investigated.
The order of the genes on the chromosome is w-m-f.
The results will not be exactly the values on the chromosome maps in textbooks. Explain why both your distances differ in the same way, and why the published distances are more accurate.

Results:

W = red eye	M = normal wing	F = straight bristles
w = white eye	m = miniature wing	f = forked bristles

Event	F₂ Male Phenotypes	Class Results
No Crossing Over	WMF	287
No Crossing Over	wmf	274
Crossing Over Between m-f	WMf	73
Crossing Over Between m-f	wmF	69
Crossing Over Between w-m	Wmf	142
Crossing Over Between w-m	wMF	136
Double Crossing Over w-m-f	WmF	25
Double Crossing Over w-m-f	wMf	21