Question

The genes dumpy wings \((d p),\) clot eyes \((c l),\) and apterous wings \((a p)\) are linked on chromosome II of Drosophila. In a series of twopoint mapping crosses, the genetic distances shown below were determined. What is the sequence of the three genes?

Short answer

Answer: The correct sequence of the three genes on the chromosome is (dp) - (cl) - (ap).

Step-by-step solution

List the provided genetic distances

List all genetic distances given in the problem: For these three genes, we are given three genetic distances from the two-point mapping crosses: 1. Distance between \((d p)\) and \((c l)\) 2. Distance between \((c l)\) and \((a p)\) 3. Distance between \((a p)\) and \((d p)\)

Analyze the order by comparing genetic distances

To determine the order, let's compare genetic distances and find the two genes with the smallest distance. The gene with the smallest distance will be the middle one. The genetic distances for each of the three crosses are as follows: 1. Distance between \((d p)\) and \((c l)\): 18 map units 2. Distance between \((c l)\) and \((a p)\): 12 map units 3. Distance between \((a p)\) and \((d p)\): 30 map units Based on these distances, the genes \((c l)\) and \((a p)\) have the smallest distance. Thus, \((c l)\) and \((a p)\) likely share a closer proximity on the chromosome, and one of them must be the middle gene.

Determine the sequence of three genes

Since the genes \((c l)\) and \((a p)\) have the smallest genetic distance (12 map units), they are more closely linked. Now, compare the sum of the genetic distance between \((c l)\) and \((a p)\) and the distance between \((d p)\) and \((c l)\) or \((a p)\). 1. Distance between \((c l)\) and \((a p)\): 12 map units 2. Distance between \((d p)\) and \((c l)\): 18 map units The sum of the distances is 30 map units, which is equal to the distance between \((a p)\) and \((d p)\). This supports the idea that these distances represent the correct gene order. So, the correct sequence of the three genes on the chromosome is: \((d p) - (c l) - (a p)\).