In the previous section, we learned how to calculate Recombination Frequency. Now, it's time to further explore how this knowledge can be used to determine the distance between two genes. This introduces a concept called a linkage map. In the present section, we will find out what a linkage map is and how we construct it. 

A linkage map, also known as a genetic map, is a chart that shows the relative positions of genes on a chromosome. The positions are determined based on recombination frequencies, which reflect the likelihood of crossover events between genes during meiosis. The map provides a visual representation of the order and relative distances between genes, which is crucial for understanding inheritance patterns, gene interactions, and chromosomal behavior. 

Step 1: Understanding Recombination Frequency

First, let's revise about recombination frequency: When genes are close together on a chromosome, they tend to be inherited together because there’s less chance of a crossover happening between them. The frequency of these crossover events is called the recombination frequency, and it's expressed as a percentage."

Example: "If two genes recombine 10% of the time, their recombination frequency is 10%. This means there’s a 10% chance that a crossover will separate these genes during meiosis."

Step 2: Calculating Recombination Frequency

Now, let’s calculate recombination frequency with an example. 

Imagine we’re studying two linked genes, A and B, in a plant. We cross plants with known genotypes and count the offspring to determine how often recombination occurs.

1. Crossing the Plants:

   • Cross plants with genotype AABB (both dominant) and aabb (both recessive).

   • F1 offspring are AaBb (heterozygous for both traits).

2. Perform a Testcross:

   • Cross the F1 heterozygous plants (AaBb) with homozygous recessive plants (aabb).

3. Count the Offspring:

   • AB (Parental): 300

   • ab (Parental): 290

   • Ab (Recombinant): 110

   • aB (Recombinant): 100

Teacher: "In this case, the total number of offspring is 800."

4. Calculate Recombination Frequency:

Recombination Frequency (RF)= Total Number of Recombinant/Total Number of Individuals*100

RF= 110+100/800*100

RF= 210/800*100

RF= 0.2625*100

RF=26.25%

So, the recombination frequency between genes A and B is 26.25%, indicating a moderate likelihood of crossover between these genes.

Step 3: Constructing the Linkage Map

Next, let’s create a linkage map. We use recombination frequencies to estimate the distances between genes. A 1% recombination frequency is equivalent to 1 map unit or centimorgan (cM)."

From The above example where the recombination frequency between genes A and B is 26.25%.  A 1% recombination frequency is equivalent to 1 map unit or centimorgan (cM)."

Convert Frequencies to Distances: Therefore, the distance between A and B is 26.25 cM and our linkage map looks like this:

Genetic Map or Linkage Map: A---------------------26.25cM------------------------------B

A map involving two genes is indeed often referred to as a Two Point Cross.