How to Create a Chromosome Map from Crossover Frequencies

Recombination: During crossing-over (prophase I of Meiosis), genes on chromosomes switch places. Crossover is random, but the likelihood that 2 genes crossover will increase if those genes are farther apart. Genes closer together are more likely to "stick together" and not switch places.

Gene Linkage Maps: Using the crossover frequencies, you can construct a map to represent the distances between genes.

This map shows chromosome #2 of Drosophila melanogaster. The distance between the genes can be written as a percentage or as a MAP UNIT. The gene for body color and and wing size are 17 map units apart.

Step 1: Start with the genes that are the farthest apart first: B and C are 45 map units apart and would be placed far apart.

B ----------------------------------------- 45% ------------------------------------------C

Step 2: Solve it like a puzzle, using a pencil to determine the positions of the other genes.