The genes dumpy (dp), clot (cl), and apterous (ap) are linked on chromosome II of Drosophila. In a series of two-point mapping crosses, the following genetic distances were determined. What is the sequence of the three genes?
dp–ap: 42
dp–cl: 3
ap–cl: 39

Verified step by step guidance

1. Identify the three genes involved: dumpy (dp), clot (cl), and apterous (ap), and note the given pairwise genetic distances: dp–ap = 42 map units, dp–cl = 3 map units, and ap–cl = 39 map units.
2. Recall that genetic distances represent the recombination frequency between two genes and that genes that are closer together have smaller distances, indicating tighter linkage.
3. Start by placing the two genes with the smallest distance on the chromosome, which are dp and cl with 3 map units apart. This suggests dp and cl are very close neighbors.
4. Next, consider the distances involving ap: dp–ap is 42 and ap–cl is 39. Since dp and cl are close, and ap is far from both, ap must be located on the chromosome such that the distances to dp and cl match the given values.
5. Arrange the genes in a linear order and verify that the sum of the smaller distances equals the largest distance, i.e., check if the distance between dp and cl plus the distance between cl and ap equals the distance between dp and ap, or if dp is between cl and ap, to determine the correct gene sequence.

Key Concepts

Genetic Linkage and Gene Mapping

Genetic linkage refers to genes located close together on the same chromosome that tend to be inherited together. Gene mapping uses recombination frequencies between linked genes to estimate their relative positions on a chromosome, measured in map units or centimorgans (cM). Lower recombination frequencies indicate closer proximity.

Two-Point Mapping

Two-point mapping involves crossing organisms to measure recombination frequency between two genes at a time. The percentage of recombinant offspring reflects the distance between those genes. By comparing distances from multiple two-point crosses, the relative order of genes can be inferred.

Determining Gene Order from Recombination Distances

To determine gene order, compare pairwise distances: the smallest distance indicates the closest genes, and the largest distances help place the third gene relative to the others. The sum of two smaller distances should approximate the largest distance if genes are in a linear sequence.