# What is a Map Unit Anyway?

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## How to Create a Chromosome Map from Crossover Frequencies

Recombination: During crossing­over (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.

## Sample Problem:

Given the crossover frequency of each of the genes on the chart, construct a chromosome map.

 Gene Frequency of Crossover A-C 30% B-C 45% B-D 40% A­-D 25%

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.

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

Step 3: Subtraction will be necessary to determine the final distances between each gene.

## Practice Problems

1. In Drosophila, bar shaped eyes (B), scalloped wings (S), Crossveinless wings (W), and Eye Color (C) are located on the X chromosome. The recombination frequency of each gene is indicated on the table. Construct a chromosome map.

 Gene Frequency of Crossover W-­B 2.5% W-C 3.0% B-C 5.5% B-S 5.5% W-S 8.0% C-S 11.0%

2. The following chart shows the crossover frequencies for genes on an autosome of the Armor Plated Squirtlesaur. Construct a chromosome map.

 Gene Frequency of Crossover P-Q 5% P-R 8% P-S 12% Q-R 13% Q-S 17%

3. Construct a map given the following data.

 Gene Frequency of Crossover A-B 24% A-C 8% C-D 2% A-F 16% F-B 8% D-F 6%

This page titled What is a Map Unit Anyway? is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Shannan Muskopf (Biology Corner) .